Use black for resize gutter in tab fullscreen mode.

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index ccdb111af04e2b3577e4343e17e03e39382b5264..4145970e5210adc9090f240606b2292fcdead6cb 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -437,6 +437,17 @@ 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();
 }