GitLab

Overview of changes:
- Added InkDropHover class that manages animating a Layer to show
  visual feedback for mouse hover.
- Wired the InkDropHover class in to the InkDropAnimationControllerImpl.
- Wired some Views up to use the hover animation.

Overview of Hover behavior:
- Hover will fade in when mouse enters an enabled View
- Hover will fade out instantly if an ink drop animation is initiated.
- Hover will fade back in, after a delay, when an ink drop animation
  completes and the mouse is still hovering the View.

TEST=InkDropHoverTest.InitialStateAfterConstruction
TEST=InkDropHoverTest.IsHoveredStateTransitions
TEST=InkDropAnimationControllerFactoryTest.HoveredStateAfterAnimateToState
TEST=HoveredStateAfterHoverTimerFiresWhenHostIsHovered.SetHoveredIsHovered
TEST=HoveredStateAfterHoverTimerFiresWhenHostIsHovered.HoveredStateAfterHoverTimerFiresWhenHostIsHovered
TEST=HoveredStateAfterHoverTimerFiresWhenHostIsHovered.HoveredStateAfterHoverTimerFiresWhenHostIsNotHovered

BUG=537238, 518919

Review URL: https://codereview.chromium.org/1390113006

Cr-Commit-Position: refs/heads/master@{#371511}