Have ExtensionHost use TabContents instead of RenderViewHost. Try #3.

chrome/browser/tab_contents/tab_contents_view_mac.h

Index: chrome/browser/tab_contents/tab_contents_view_mac.h
diff --git a/chrome/browser/tab_contents/tab_contents_view_mac.h b/chrome/browser/tab_contents/tab_contents_view_mac.h
index 406e221e877af7cf51e91e921890aa259a2ac094..bc3b5955c50f509b72bc863e62bfdb2d5426b7f8 100644
--- a/chrome/browser/tab_contents/tab_contents_view_mac.h
+++ b/chrome/browser/tab_contents/tab_contents_view_mac.h
@@ -42,6 +42,11 @@ class Point;
 
 // Expose this, since sometimes one needs both the NSView and the TabContents.
 - (TabContents*)tabContents;
+
+// Set frame, then notify the RenderWidgetHost that the frame has been changed,
+// but do it in a separate task, using |performSelector:withObject:afterDelay:|.
+// This stops the flickering issue in http://crbug.com/31970
+- (void)setFrameWithDeferredUpdate:(NSRect)frame;
 @end
 
 // Mac-specific implementation of the TabContentsView. It owns an NSView that