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..ce6ea32b5c014b0713a6c4e495e023a85c361186 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -410,8 +410,8 @@ 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) {
-    // Android WebView uses system scrollbars, so make ours invisible.
+  // Also hide scrollbars for Android WebView, since it uses system scrollbars.
+  if (ui::ShouldHideScrollbars() || using_synchronous_compositor) {
     settings.scrollbar_animator = cc::LayerTreeSettings::NO_ANIMATOR;
     settings.solid_color_scrollbar_color = SK_ColorTRANSPARENT;
   } else {
@@ -455,7 +455,10 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
 
 #else  // defined(OS_ANDROID)
 #if !defined(OS_MACOSX)
-  if (ui::IsOverlayScrollbarEnabled()) {
+  if (ui::ShouldHideScrollbars()) {
+    settings.scrollbar_animator = cc::LayerTreeSettings::NO_ANIMATOR;
+    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 {