Fix cc scrollbar layer issues with initialization, and use element ids throughout.

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index 14dfe30a5ef5312c4bc693260022cea074e493c5..6d504cecdc40903608d9b35336909c739504fdc4 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -16,9 +16,10 @@ ScrollbarAnimationController::CreateScrollbarAnimationControllerAndroid(
     ElementId scroll_element_id,
     ScrollbarAnimationControllerClient* client,
     base::TimeDelta fade_delay,
-    base::TimeDelta fade_duration) {
+    base::TimeDelta fade_duration,
+    float initial_opacity) {
   return base::WrapUnique(new ScrollbarAnimationController(
-      scroll_element_id, client, fade_delay, fade_duration));
+      scroll_element_id, client, fade_delay, fade_duration, initial_opacity));
 }
 
 std::unique_ptr<ScrollbarAnimationController>
@@ -27,16 +28,19 @@ ScrollbarAnimationController::CreateScrollbarAnimationControllerAuraOverlay(
     ScrollbarAnimationControllerClient* client,
     base::TimeDelta fade_delay,
     base::TimeDelta fade_duration,
-    base::TimeDelta thinning_duration) {
+    base::TimeDelta thinning_duration,
+    float initial_opacity) {
   return base::WrapUnique(new ScrollbarAnimationController(
-      scroll_element_id, client, fade_delay, fade_duration, thinning_duration));
+      scroll_element_id, client, fade_delay, fade_duration, thinning_duration,
+      initial_opacity));
 }
 
 ScrollbarAnimationController::ScrollbarAnimationController(
     ElementId scroll_element_id,
     ScrollbarAnimationControllerClient* client,
     base::TimeDelta fade_delay,
-    base::TimeDelta fade_duration)
+    base::TimeDelta fade_duration,
+    float initial_opacity)
     : client_(client),
       fade_delay_(fade_delay),
       fade_duration_(fade_duration),
@@ -46,19 +50,18 @@ ScrollbarAnimationController::ScrollbarAnimationController(
       scroll_element_id_(scroll_element_id),
       currently_scrolling_(false),
       show_in_fast_scroll_(false),
-      opacity_(0.0f),
+      opacity_(initial_opacity),
       show_scrollbars_on_scroll_gesture_(false),
       need_thinning_animation_(false),
-      weak_factory_(this) {
-  ApplyOpacityToScrollbars(0.0f);
-}
+      weak_factory_(this) {}
 
 ScrollbarAnimationController::ScrollbarAnimationController(
     ElementId scroll_element_id,
     ScrollbarAnimationControllerClient* client,
     base::TimeDelta fade_delay,
     base::TimeDelta fade_duration,
-    base::TimeDelta thinning_duration)
+    base::TimeDelta thinning_duration,
+    float initial_opacity)
     : client_(client),
       fade_delay_(fade_delay),
       fade_duration_(fade_duration),
@@ -68,7 +71,7 @@ ScrollbarAnimationController::ScrollbarAnimationController(
       scroll_element_id_(scroll_element_id),
       currently_scrolling_(false),
       show_in_fast_scroll_(false),
-      opacity_(0.0f),
+      opacity_(initial_opacity),
       show_scrollbars_on_scroll_gesture_(true),
       need_thinning_animation_(true),
       weak_factory_(this) {
@@ -78,8 +81,6 @@ ScrollbarAnimationController::ScrollbarAnimationController(
   horizontal_controller_ = SingleScrollbarAnimationControllerThinning::Create(
       scroll_element_id, ScrollbarOrientation::HORIZONTAL, client,
       thinning_duration);
-
-  ApplyOpacityToScrollbars(0.0f);
 }
 
 ScrollbarAnimationController::~ScrollbarAnimationController() {}