canvas2d: Flush skia for CanvasResourceProviderSharedImage.

It is important to flush skia's GPU work before using the resource for
compositing. Woops.

R=fserb@chromium.org

Change-Id: I71c0966719fa022352a52c39d2a2ce74330c1c33
Reviewed-on: https://chromium-review.googlesource.com/c/1487146
Auto-Submit: Khushal <khushalsagar@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635675}

third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc

@@ -502,6 +502,7 @@ class CanvasResourceProviderSharedImage : public CanvasResourceProvider {
   scoped_refptr<CanvasResource> ProduceCanvasResource() override {
     TRACE_EVENT0("blink",
                  "CanvasResourceProviderSharedImage::ProduceCanvasResource");
+    FlushSkia();
 
     scoped_refptr<CanvasResource> resource_snapshot = resource_;
 
@@ -514,6 +515,7 @@ class CanvasResourceProviderSharedImage : public CanvasResourceProvider {
 
   scoped_refptr<StaticBitmapImage> Snapshot() override {
     TRACE_EVENT0("blink", "CanvasResourceProviderSharedImage::Snapshot");
+    FlushSkia();
 
     scoped_refptr<CanvasResource> resource_snapshot = resource_;