Fix viewport computation in Mac UC

content/browser/renderer_host/render_widget_host_view_mac.mm

 // Copyright (c) 2012 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.
 
 #include "content/browser/renderer_host/render_widget_host_view_mac.h"
 
 #import <objc/runtime.h>
 #include <OpenGL/gl.h>
 #include <QuartzCore/QuartzCore.h>
 
@@ skipping 1795 matching lines @@
   }
   return false;
 }
 
 void RenderWidgetHostViewMac::OnSwapCompositorFrame(
     uint32 output_surface_id, scoped_ptr<cc::CompositorFrame> frame) {
   TRACE_EVENT0("browser", "RenderWidgetHostViewMac::OnSwapCompositorFrame");
 
   if (frame->delegated_frame_data) {
     float scale_factor = frame->metadata.device_scale_factor;
-    gfx::Size dip_size = ToCeiledSize(frame->metadata.viewport_size);
-    gfx::Size pixel_size = ConvertSizeToPixel(
-        scale_factor, dip_size);
+
+    // Compute the frame size based on the root render pass rect size.
+    cc::RenderPass* root_pass =
+        frame->delegated_frame_data->render_pass_list.back();
+    gfx::Size pixel_size = root_pass->output_rect.size();
+    gfx::Size dip_size =
+        ConvertSizeToDIP(scale_factor, pixel_size);
+
     root_layer_->SetBounds(gfx::Rect(dip_size));
-
     if (!render_widget_host_->is_hidden()) {
       EnsureBrowserCompositorView();
       browser_compositor_view_->GetCompositor()->SetScaleAndSize(
           scale_factor, pixel_size);
     }
 
     SendVSyncParametersToRenderer();
 
     delegated_frame_host_->SwapDelegatedFrame(
         output_surface_id,
@@ skipping 2119 matching lines @@
 
 // "-webkit-app-region: drag | no-drag" is implemented on Mac by excluding
 // regions that are not draggable. (See ControlRegionView in
 // native_app_window_cocoa.mm). This requires the render host view to be
 // draggable by default.
 - (BOOL)mouseDownCanMoveWindow {
   return YES;
 }
 
 @end