Make copySurface work for texture dsts, return a bool, & add unit test.

src/gpu/SkGrPixelRef.cpp

Index: src/gpu/SkGrPixelRef.cpp
diff --git a/src/gpu/SkGrPixelRef.cpp b/src/gpu/SkGrPixelRef.cpp
index 58f516a19c6d3ad370456fcb0ad8a24fcd23b182..b8f243a3f40bde7c7ffed8c7bf631eff318f17be 100644
--- a/src/gpu/SkGrPixelRef.cpp
+++ b/src/gpu/SkGrPixelRef.cpp
@@ -83,10 +83,10 @@ static SkGrPixelRef* copy_to_new_texture_pixelref(GrTexture* texture, SkColorTyp
     }
 
     // Blink is relying on the above copy being sent to GL immediately in the case when the source
-    // is a WebGL canvas backing store. We could have a TODO to remove this flush flag, but we have
+    // is a WebGL canvas backing store. We could have a TODO to remove this flush, but we have
     // a larger TODO to remove SkGrPixelRef entirely.
-    context->copySurface(dst->asRenderTarget(), texture, srcRect, SkIPoint::Make(0,0),
-                         GrContext::kFlushWrites_PixelOp);
+    context->copySurface(dst, texture, srcRect, SkIPoint::Make(0,0));
+    context->flushSurfaceWrites(dst);
 
     SkImageInfo info = SkImageInfo::Make(desc.fWidth, desc.fHeight, dstCT, kPremul_SkAlphaType,
                                          dstPT);