Fix WebGL PushFrame crash when GetDrawingBuffer returns NULL

Bug: 1059491
Change-Id: I6df4870cb8ece3986a2ad0fecd28b50593ae2e8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108874Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>
Cr-Commit-Position: refs/heads/master@{#751547}

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -1412,11 +1412,11 @@ void WebGLRenderingContextBase::DidDraw() {
 }
 
 bool WebGLRenderingContextBase::PushFrame() {
-  int width = GetDrawingBuffer()->Size().Width();
-  int height = GetDrawingBuffer()->Size().Height();
   int submitted_frame = false;
   if (PaintRenderingResultsToCanvas(kBackBuffer)) {
     if (Host()->GetOrCreateCanvasResourceProvider(kPreferAcceleration)) {
+      int width = GetDrawingBuffer()->Size().Width();
+      int height = GetDrawingBuffer()->Size().Height();
       submitted_frame =
           Host()->PushFrame(Host()->ResourceProvider()->ProduceCanvasResource(),
                             SkIRect::MakeWH(width, height));