fuzzer fixes

Fix path bugs exposed by the path fuzzer.

Changes to existing gm and samplecode files defer their calls to construct
SkPath objects until the first draw instead of at test initialization.

Add an experimental call to SkPath to validate the internal SkPathRef.

Fix SkPath::addPoly to set the last moveto after adding a close verb.

Fix stroke to handle failures when computing the unit normal.

Add a unit test for the unit normal failure.

R=reed@google.com

Review URL: https://codereview.chromium.org/953383002
diff --git a/gm/circularclips.cpp b/gm/circularclips.cpp
index ec766fb..614ebb6 100644
--- a/gm/circularclips.cpp
+++ b/gm/circularclips.cpp
@@ -13,8 +13,8 @@
     SkScalar fX1, fX2, fY, fR;
     SkPath   fCircle1, fCircle2;
 
-public:
-    CircularClipsGM() {
+protected:
+    void onOnceBeforeDraw() SK_OVERRIDE {
         fX1 = 80;
         fX2 = 120;
         fY = 50;
@@ -24,7 +24,6 @@
         fCircle2.addCircle(fX2, fY, fR, SkPath::kCW_Direction);
     }
 
-protected:
 
     bool runAsBench() const SK_OVERRIDE { return true; }