Use black for resize gutter in tab fullscreen mode.

content/browser/renderer_host/delegated_frame_host.cc

Index: content/browser/renderer_host/delegated_frame_host.cc
diff --git a/content/browser/renderer_host/delegated_frame_host.cc b/content/browser/renderer_host/delegated_frame_host.cc
index cef537d39b0e64fd7b7137667b296338eaa1b043..109c4a0ca59f183a1c14ec4f123687d65f47177c 100644
--- a/content/browser/renderer_host/delegated_frame_host.cc
+++ b/content/browser/renderer_host/delegated_frame_host.cc
@@ -256,16 +256,26 @@ void DelegatedFrameHost::WasResized() {
   UpdateGutters();
 }
 
+SkColor DelegatedFrameHost::GetGutterColor() 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.
+  SkColor color = background_color_;
+  client_->DelegatedFrameHostOverrideGutterColor(&color);

[danakj, 2016/04/25 22:17:58]

nit: i think maybe passing the color and returning a possibly-changed color is a
bit nicer and may produce nicer codegen? no pointers, RVO, etc.

then this would be

return client_->DFHGetGutterColor(color);

+  return color;
+}
+
 void DelegatedFrameHost::UpdateGutters() {
   if (surface_id_.is_null()) {
     right_gutter_.reset();
     bottom_gutter_.reset();
     return;
   }
+
   if (current_frame_size_in_dip_.width() <
       client_->DelegatedFrameHostDesiredSizeInDIP().width()) {
     right_gutter_.reset(new ui::Layer(ui::LAYER_SOLID_COLOR));
-    right_gutter_->SetColor(background_color_);
+    right_gutter_->SetColor(GetGutterColor());
     int width = client_->DelegatedFrameHostDesiredSizeInDIP().width() -
                 current_frame_size_in_dip_.width();
     // The right gutter also includes the bottom-right corner, if necessary.
@@ -281,7 +291,7 @@ void DelegatedFrameHost::UpdateGutters() {
   if (current_frame_size_in_dip_.height() <
       client_->DelegatedFrameHostDesiredSizeInDIP().height()) {
     bottom_gutter_.reset(new ui::Layer(ui::LAYER_SOLID_COLOR));
-    bottom_gutter_->SetColor(background_color_);
+    bottom_gutter_->SetColor(GetGutterColor());
     int width = current_frame_size_in_dip_.width();
     int height = client_->DelegatedFrameHostDesiredSizeInDIP().height() -
                  current_frame_size_in_dip_.height();