Add flag to hide scrollbars completely.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 6e676358bda2c06bf526634b9546a4e433908733..fee978164ce2cf998441eb2e8d90870da37ad659 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -410,7 +410,7 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
   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;
@@ -455,7 +455,10 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
 
 #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 {