Merge Compositor's ScrollbarAnimationControllers into single class

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 ae598c0253cc9674c4c93b2accfc9f45b6d54144..36ca8fd7c25b550e99de6ae7811b1193553db8f0 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -388,7 +388,7 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
     settings.scrollbar_animator = cc::LayerTreeSettings::NO_ANIMATOR;
     settings.solid_color_scrollbar_color = SK_ColorTRANSPARENT;
   } else {
-    settings.scrollbar_animator = cc::LayerTreeSettings::LINEAR_FADE;
+    settings.scrollbar_animator = cc::LayerTreeSettings::ANDROID_OVERLAY;
     settings.scrollbar_fade_delay = base::TimeDelta::FromMilliseconds(300);
     settings.scrollbar_fade_resize_delay =
         base::TimeDelta::FromMilliseconds(2000);
@@ -427,7 +427,7 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
 #else  // defined(OS_ANDROID)
 #if !defined(OS_MACOSX)
   if (ui::IsOverlayScrollbarEnabled()) {
-    settings.scrollbar_animator = cc::LayerTreeSettings::THINNING;
+    settings.scrollbar_animator = cc::LayerTreeSettings::AURA_OVERLAY;
     settings.scrollbar_fade_delay = ui::kOverlayScrollbarFadeOutDelay;
     settings.scrollbar_fade_resize_delay =
         ui::kOverlayScrollbarFadeOutDelay;
@@ -438,7 +438,7 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
   } else {
     // TODO(bokan): This section is probably unneeded? We don't use scrollbar
     // animations for non overlay scrollbars.
-    settings.scrollbar_animator = cc::LayerTreeSettings::LINEAR_FADE;
+    settings.scrollbar_animator = cc::LayerTreeSettings::ANDROID_OVERLAY;

[ncarter (slow), 2017/02/17 20:47:17]

As a result of the renaming, this looks weirder than before, because we're using
an ANDROID_ constant inside a !defined(OS_ANDROID) #ifdef context.

I don't particularly mind either style of name, but as someone who isn't
familiar with this code, I found it much easier to understand the original
THINNING/LINEAR_FADE names.

lgtm because I don't want to bikeshed, but maybe you could add comments to the
enum declarations like: // Android-style linear fade animation ?

[chaopeng, 2017/02/17 21:35:58]

We have a plan to fix Mac scrollbar and this TODO soon. 

[bokan, 2017/02/21 15:49:56]

This is already for non-Mac. I think we could just remove this section without
breaking anything. Could you put up a patch?

     settings.solid_color_scrollbar_color = SkColorSetARGB(128, 128, 128, 128);
     settings.scrollbar_fade_delay = base::TimeDelta::FromMilliseconds(500);
     settings.scrollbar_fade_resize_delay =