Introduce bottom controls to CC and let it respond to scrolling

cc/ipc/compositor_frame_metadata.mojom

 // Copyright 2016 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.
 
 module cc.mojom;
 
 import "cc/ipc/selection.mojom";
 import "cc/ipc/surface_id.mojom";
 import "ui/events/mojo/latency_info.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
 // See cc/output/compositor_frame_metadata.h.
 struct CompositorFrameMetadata {
   float device_scale_factor;
   gfx.mojom.Vector2dF root_scroll_offset;
   float page_scale_factor;
   gfx.mojom.SizeF scrollable_viewport_size;
   gfx.mojom.SizeF root_layer_size;
   float min_page_scale_factor;
   float max_page_scale_factor;
   bool root_overflow_x_hidden;
   bool root_overflow_y_hidden;
-  gfx.mojom.Vector2dF location_bar_offset;
-  gfx.mojom.Vector2dF location_bar_content_translation;
+  gfx.mojom.Vector2dF top_bar_translate;
+  gfx.mojom.Vector2dF top_bar_shown;
+  gfx.mojom.Vector2dF bottom_bar_translate;
+  gfx.mojom.Vector2dF bottom_bar_shown;
   uint32 root_background_color;
   Selection selection;
   array<ui.mojom.LatencyInfo> latency_info;
   array<uint32> satisfies_sequences;
   array<SurfaceId> referenced_surfaces;
 };