cc: flush the drawing before unlocking the filtered source

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index 1af26c1cc7f6fe579aaad98664408a911201a667..2b20d25d39f18f17a362c4b0d88c4efa9d506731 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -655,7 +655,11 @@ static skia::RefPtr<SkImage> ApplyImageFilter(
   surface->getCanvas()->translate(-dst_rect.x(), -dst_rect.y());
   surface->getCanvas()->drawImage(srcImage.get(), src_rect.x(), src_rect.y(),
                                   &paint);
-
+  // Flush the drawing before source texture read lock goes out of scope.
+  // Skia API does not guarantee that when the SkImage goes out of scope,
+  // its externally referenced resources would force the rendering to be
+  // flushed.
+  surface->getCanvas()->flush();

[Kimmo Kinnunen, 2016/02/11 18:51:45]

Note: naturally this trades off msaa perf for correctness.
Most of the perf regression could be gained back with Brian's envisioned
SkSurface::flushButNotResolve() and SkSurface::prepareForExternalIO() API
additions. 

[ericrk, 2016/02/11 22:03:20]

Ok... Now I understand what's going on here a bit better... We need this change
independent of the other one. Thanks for explaining!

   skia::RefPtr<SkImage> image = skia::AdoptRef(surface->newImageSnapshot());
   if (!image || !image->isTextureBacked()) {
     return skia::RefPtr<SkImage>();