Always postConcat() pipeline gradient matrices

In order to unblock https://skia-review.googlesource.com/c/26564 &
https://skia-review.googlesource.com/c/26427, switch all impls to use a
single/atomic postConcat for the gradient matrix.

This is much easier to guard than the changes above.

Change-Id: Ib72dbc7b5a56a96390f4468fd12caf8b6a0e46ef
Reviewed-on: https://skia-review.googlesource.com/26620
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/src/shaders/gradients/SkSweepGradient.cpp b/src/shaders/gradients/SkSweepGradient.cpp
index f984542..0a1d83e 100644
--- a/src/shaders/gradients/SkSweepGradient.cpp
+++ b/src/shaders/gradients/SkSweepGradient.cpp
@@ -222,7 +222,11 @@
                                                   SkMatrix* matrix,
                                                   SkRasterPipeline* p,
                                                   SkRasterPipeline*) const {
+#ifdef SK_SUPPORT_LEGACY_GRADIENT_MATRIX_MATH
     matrix->postTranslate(-fCenter.fX, -fCenter.fY);
+#else
+    matrix->postConcat(fPtsToUnit);
+#endif
     p->append(SkRasterPipeline::xy_to_unit_angle);
 
     return true;