Add flag to hide scrollbars completely.

content/renderer/gpu/render_widget_compositor.cc

 // Copyright (c) 2013 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/renderer/gpu/render_widget_compositor.h"
 
 #include <stddef.h>
 #include <limits>
 #include <string>
 #include <utility>
@@ skipping 392 matching lines @@
 
 #if defined(OS_ANDROID)
   bool using_synchronous_compositor =
       GetContentClient()->UsingSynchronousCompositing();
 
   // We can't use GPU rasterization on low-end devices, because the Ganesh
   // cache would consume too much memory.
   if (base::SysInfo::IsLowEndDevice())
     settings.gpu_rasterization_enabled = false;
   settings.using_synchronous_renderer_compositor = using_synchronous_compositor;
-  if (using_synchronous_compositor) {
+  if (using_synchronous_compositor || ui::IsHiddenScrollbarEnabled()) {
     // Android WebView uses system scrollbars, so make ours invisible.

[skobes, 2016/08/02 17:55:55]

Update this comment since there is a new reason for us to be in this path.

[Eric Seckler, 2016/08/03 17:34:54]

Done.

     settings.scrollbar_animator = cc::LayerTreeSettings::NO_ANIMATOR;
     settings.solid_color_scrollbar_color = SK_ColorTRANSPARENT;
   } else {
     settings.scrollbar_animator = cc::LayerTreeSettings::LINEAR_FADE;
     settings.scrollbar_fade_delay_ms = 300;
     settings.scrollbar_fade_resize_delay_ms = 2000;
     settings.scrollbar_fade_duration_ms = 300;
     settings.solid_color_scrollbar_color = SkColorSetARGB(128, 128, 128, 128);
   }
   settings.renderer_settings.highp_threshold_min = 2048;
@@ skipping 23 matching lines @@
   settings.renderer_settings.should_clear_root_render_pass =
       !using_synchronous_compositor;
 
   // TODO(danakj): Only do this on low end devices.
   settings.create_low_res_tiling = true;
 
   settings.use_external_begin_frame_source = true;
 
 #else  // defined(OS_ANDROID)
 #if !defined(OS_MACOSX)
-  if (ui::IsOverlayScrollbarEnabled()) {
+  if (ui::IsHiddenScrollbarEnabled()) {
+    settings.scrollbar_animator = cc::LayerTreeSettings::NO_ANIMATOR;

[skobes, 2016/08/02 17:55:55]

Note that this implementation only hides native composited scrollbars.  It will
not hide custom scrollbars, or native scrollbars on non-composited scrollers. 
If you care about those cases you will need some Blink side plumbing.

[Eric Seckler, 2016/08/03 17:34:54]

I think just supporting native composited scrollbars is a good first step. We'll
want to hide native non-composited ones next, but I'm happy to do that in a
later patch. Hiding custom scrollbars is only a nice-to-have.

+    settings.solid_color_scrollbar_color = SK_ColorTRANSPARENT;
+  } else if (ui::IsOverlayScrollbarEnabled()) {
     settings.scrollbar_animator = cc::LayerTreeSettings::THINNING;
     settings.solid_color_scrollbar_color = SkColorSetARGB(128, 128, 128, 128);
   } else {
     settings.scrollbar_animator = cc::LayerTreeSettings::LINEAR_FADE;
     settings.solid_color_scrollbar_color = SkColorSetARGB(128, 128, 128, 128);
   }
   settings.scrollbar_fade_delay_ms = 500;
   settings.scrollbar_fade_resize_delay_ms = 500;
   settings.scrollbar_fade_duration_ms = 300;
 #endif  // !defined(OS_MACOSX)
@@ skipping 673 matching lines @@
 
   remote_proto_channel_receiver_->OnProtoReceived(std::move(deserialized));
 }
 
 void RenderWidgetCompositor::SetPaintedDeviceScaleFactor(
     float device_scale) {
   layer_tree_host_->SetPaintedDeviceScaleFactor(device_scale);
 }
 
 }  // namespace content