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

chrome/browser/tab_contents/tab_contents_view_mac.mm

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Carbon/Carbon.h>
 
 #include "chrome/browser/tab_contents/tab_contents_view_mac.h"
 
 #include <string>
 
@@ skipping 38 matching lines @@
 - (void)registerDragTypes;
 - (void)setCurrentDragOperation:(NSDragOperation)operation;
 - (void)startDragWithDropData:(const WebDropData&)dropData
             dragOperationMask:(NSDragOperation)operationMask
                         image:(NSImage*)image
                        offset:(NSPoint)offset;
 - (void)cancelDeferredClose;
 - (void)clearTabContentsView;
 - (void)closeTabAfterEvent;
 - (void)viewDidBecomeFirstResponder:(NSNotification*)notification;
+// Notify the RenderWidgetHost that the frame was updated so it can resize
+// its contents.
+- (void)renderWidgetHostWasResized;
 @end
 
 namespace tab_contents_view_mac {
 TabContentsView* CreateTabContentsView(TabContents* tab_contents) {
   return new TabContentsViewMac(tab_contents);
 }
 }
 
 TabContentsViewMac::TabContentsViewMac(TabContents* tab_contents)
     : tab_contents_(tab_contents),
       preferred_width_(0),
       overlaid_view_(nil) {
 }
 
 TabContentsViewMac::~TabContentsViewMac() {
   // This handles the case where a renderer close call was deferred
   // while the user was operating a UI control which resulted in a
   // close.  In that case, the Cocoa view outlives the
   // TabContentsViewMac instance due to Cocoa retain count.
   [cocoa_view_ cancelDeferredClose];
   [cocoa_view_ clearTabContentsView];
 }
 
 void TabContentsViewMac::CreateView(const gfx::Size& initial_size) {
   TabContentsViewCocoa* view =
       [[TabContentsViewCocoa alloc] initWithTabContentsViewMac:this];
-  [view setAutoresizingMask:(NSViewWidthSizable | NSViewHeightSizable)];
   cocoa_view_.reset(view);
 }
 
 RenderWidgetHostView* TabContentsViewMac::CreateViewForWidget(
     RenderWidgetHost* render_widget_host) {
   if (render_widget_host->view()) {
     // During testing, the view will already be set up in most cases to the
     // test view, so we don't want to clobber it with a real one. To verify that
     // this actually is happening (and somebody isn't accidentally creating the
     // view twice), we check for the RVH Factory, which will be set when we're
@@ skipping 405 matching lines @@
       initWithContents:[self tabContents]
                   view:self
               dropData:&dropData
                  image:image
                 offset:offset
             pasteboard:[NSPasteboard pasteboardWithName:NSDragPboard]
      dragOperationMask:operationMask]);
   [dragSource_ startDrag];
 }
 
+- (void)setFrameWithDeferredUpdate:(NSRect)frameRect {
+  [super setFrame:frameRect];
+  [self performSelector:@selector(renderWidgetHostWasResized)
+             withObject:nil
+             afterDelay:0];
+}
+
+- (void)renderWidgetHostWasResized {
+  TabContents* tabContents = [self tabContents];
+  if (tabContents->render_view_host())
+    tabContents->render_view_host()->WasResized();
+}
+
 // NSDraggingSource methods
 
 // Returns what kind of drag operations are available. This is a required
 // method for NSDraggingSource.
 - (NSDragOperation)draggingSourceOperationMaskForLocal:(BOOL)isLocal {
   if (dragSource_.get())
     return [dragSource_ draggingSourceOperationMaskForLocal:isLocal];
   // No web drag source - this is the case for dragging a file from the
   // downloads manager. Default to copy operation. Note: It is desirable to
   // allow the user to either move or copy, but this requires additional
@@ skipping 70 matching lines @@
       [[[notification userInfo] objectForKey:kSelectionDirection]
         unsignedIntegerValue];
   if (direction == NSDirectSelection)
     return;
 
   [self tabContents]->
       FocusThroughTabTraversal(direction == NSSelectingPrevious);
 }
 
 @end