Fix Null-dereference READ in blink::WebGLRenderingContextBase::commit

The drawing buffer is not ready when blink::WebGLRenderingContextBase::commit
is called. Added additional check for drawing buffer.

Bug: 976830
Change-Id: I810d5ad7c847ff5f5aa3ff850f812cfbd3dbc750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692827Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676163}

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

@@ -743,6 +743,8 @@ ImageBitmap* WebGLRenderingContextBase::TransferToImageBitmapBase(
 }
 
 void WebGLRenderingContextBase::commit() {
+  if (!GetDrawingBuffer())
+    return;
   int width = GetDrawingBuffer()->Size().Width();
   int height = GetDrawingBuffer()->Size().Height();