Reland "Improve degenerate 2pt conical gradient cases"

This reverts commit 95af4726bf91669e51250ebd3baa2925b6975e8e.

Reason for revert: I think this may not have been the reason the Android roll was failing.  We've rolled, so it's a good time to try again.

Original change's description:
> Revert "Improve degenerate 2pt conical gradient cases"
> 
> This reverts commit 879dab87ab78613458f9d36f843bc3c6ffdafc74.
> 
> Reason for revert: Android roll failed.
> https://sponge.corp.google.com/target?id=93bc6b8d-9b42-4805-b204-46ae62f1b005&target=x86+CtsGraphicsTestCases&searchFor=&show=FAILED&sortBy=STATUS
> A test VectorDrawableTest.testVectorDrawableGradient fails.
> 
> Original change's description:
> > Improve degenerate 2pt conical gradient cases
> > 
> > This was originally a reland of "Fix div-by-zero loophole in gradient factory func", c34dd6c5263490b94ef9af7a14dee1b4bc872b58, but:
> > 
> > The change caused blink layout tests when encountering very small or zero radii. The original patch switched the order of checking if the radii are equal and if the start radius was 0. In the case where both radii are 0, the original code created an actual radial gradient of radius 0 and the new code rejected the shader. A radial gradient with radius of 0 properly renders the last border color as a fill.
> > 
> > This made me realize that the case when the center positions and the radii are the same can be handled more correctly than just always returning an empty shader, so the fix now applies simplifications to the gradient definition depending on the tile mode and should not trigger any blink tests. I added a row to the gradient edge cases GM to make sure it degrades gracefully.
> > 
> > Original change's description:
> > > Fix div-by-zero loophole in gradient factory func
> > >
> > > Bug: oss-fuzz:10373
> > > Change-Id: I4277fb63e3186ee34feaf09ecf6aeddeb532f9c1
> > > Reviewed-on: https://skia-review.googlesource.com/c/168269
> > > Reviewed-by: Kevin Lubick <kjlubick@google.com>
> > > Commit-Queue: Michael Ludwig <michaelludwig@google.com>
> > 
> > Docs-Preview: https://skia.org/?cl=168487
> > Bug: oss-fuzz:10373
> > Change-Id: Ib0a6e7f807560a5dcf24d1c8e0146817af2d9606
> > Reviewed-on: https://skia-review.googlesource.com/c/168487
> > Reviewed-by: Mike Reed <reed@google.com>
> > Reviewed-by: Florin Malita <fmalita@chromium.org>
> > Commit-Queue: Michael Ludwig <michaelludwig@google.com>
> 
> TBR=caryclark@google.com,fmalita@chromium.org,fmalita@google.com,reed@google.com,michaelludwig@google.com
> 
> Change-Id: I91b896c4a438c02206679b327a01b47f40993965
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: oss-fuzz:10373
> Reviewed-on: https://skia-review.googlesource.com/c/170272
> Reviewed-by: Stan Iliev <stani@google.com>
> Commit-Queue: Stan Iliev <stani@google.com>

TBR=caryclark@google.com,fmalita@chromium.org,fmalita@google.com,reed@google.com,stani@google.com,michaelludwig@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: oss-fuzz:10373
Change-Id: I7577fcea9eb8a875e94723ab2cca2fcc990b82b2
Reviewed-on: https://skia-review.googlesource.com/c/170279
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/shaders/gradients/SkTwoPointConicalGradient.cpp b/src/shaders/gradients/SkTwoPointConicalGradient.cpp
index 49d0631..469c8ad 100644
--- a/src/shaders/gradients/SkTwoPointConicalGradient.cpp
+++ b/src/shaders/gradients/SkTwoPointConicalGradient.cpp
@@ -55,8 +55,10 @@
     Type     gradientType;
 
     if (SkScalarNearlyZero((c0 - c1).length())) {
-        if (SkScalarNearlyZero(SkTMax(r0, r1))) {
-            return nullptr; // Degenerate case; avoid dividing by zero.
+        if (SkScalarNearlyZero(SkTMax(r0, r1)) || SkScalarNearlyEqual(r0, r1)) {
+            // Degenerate case; avoid dividing by zero. Should have been caught by caller but
+            // just in case, recheck here.
+            return nullptr;
         }
         // Concentric case: we can pretend we're radial (with a tiny twist).
         const SkScalar scale = sk_ieee_float_divide(1, SkTMax(r0, r1));