Use black for resize gutter in tab fullscreen mode.

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>
 #include <stdint.h>
@@ skipping 419 matching lines @@
 // DelegatedFrameHost, public:
 
 ui::Layer* RenderWidgetHostViewMac::DelegatedFrameHostGetLayer() const {
   return root_layer_.get();
 }
 
 bool RenderWidgetHostViewMac::DelegatedFrameHostIsVisible() const {
   return !render_widget_host_->is_hidden();
 }
 
+void RenderWidgetHostViewMac::DelegatedFrameHostOverrideGutterColor(
+    SkColor* color) const {
+  // In fullscreen mode resizing is uncommon, so it makes more sense to make
+  // the initial switch to fullscreen mode look better by using black as the
+  // gutter color.
+  if (render_widget_host_->delegate()->IsFullscreenForCurrentTab(
+          render_widget_host_)) {
+    *color = SK_ColorBLACK;
+  }
+}
+
 gfx::Size RenderWidgetHostViewMac::DelegatedFrameHostDesiredSizeInDIP() const {
   return GetViewBounds().size();
 }
 
 bool RenderWidgetHostViewMac::DelegatedFrameCanCreateResizeLock() const {
   // Mac uses the RenderWidgetResizeHelper instead of a resize lock.
   return false;
 }
 
 std::unique_ptr<ResizeLock>
@@ skipping 2945 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