Disable partial swaps for webview guest renderer until we can figure out how to do that properly.

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index 653564e0d482d12b95345643ae51aa8a11274afb..717cef484803e0b08dba815cc9be5374577e6b1f 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -527,6 +527,11 @@ class CONTENT_EXPORT RenderWidget
   // Cached on the RenderWidget because determining support is costly.
   bool using_asynchronous_swapbuffers_;
 
+  // Specifies whether partially swapping composited buffers is
+  // allowed for a renderer. Partial swaps will be used if they are both
+  // allowed and supported.
+  bool allow_partial_swap_;
+
   // Number of OnSwapBuffersComplete we are expecting. Incremented each time
   // WebWidget::composite has been been performed when the RenderWidget subclass
   // SupportsAsynchronousSwapBuffers. Decremented in OnSwapBuffers. Will block