Move gesture nav arrow to its own layer

Currently, gesture nav affordance uses a single layer that paints
everything in each frame. This CL moves the arrow to its own layer which
is drawn once and then appropriate transform and opacity are applied on
it. This is needed for implementing pull-to-refresh which needs the
arrow be rotated. Also, generally we want to decompose the affordance
into a few layers to avoid painting it on each frame.

BUG=702404,743239
TEST=none

Change-Id: I9140471d6b6edbbb84f97fd5148994fea58e5a90
Reviewed-on: https://chromium-review.googlesource.com/594840Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491261}

content/browser/web_contents/aura/gesture_nav_simple.h

@@ -27,9 +27,6 @@ class GestureNavSimple : public OverscrollControllerDelegate {
   void OnAffordanceAnimationEnded();
 
  private:
-  void AbortGestureAnimation();
-  void CompleteGestureAnimation();
-
   // OverscrollControllerDelegate:
   gfx::Size GetDisplaySize() const override;
   bool OnOverscrollUpdate(float delta_x, float delta_y) override;
@@ -39,14 +36,14 @@ class GestureNavSimple : public OverscrollControllerDelegate {
                               OverscrollSource source) override;
   base::Optional<float> GetMaxOverscrollDelta() const override;
 
-  WebContentsImpl* web_contents_;
+  WebContentsImpl* web_contents_ = nullptr;
   std::unique_ptr<Affordance> affordance_;
-  float completion_threshold_;
+  float completion_threshold_ = 0.f;
   OverscrollSource source_ = OverscrollSource::NONE;
 
   // When an overscroll is active, represents the maximum overscroll delta we
   // expect in OnOverscrollUpdate().
-  float max_delta_;
+  float max_delta_ = 0.f;
 
   DISALLOW_COPY_AND_ASSIGN(GestureNavSimple);
 };