[ash-md] Fixed ShelfBackgroundAnimator to not snap target values when being preempted.

ash/common/shelf/shelf_background_animator.cc

Index: ash/common/shelf/shelf_background_animator.cc
diff --git a/ash/common/shelf/shelf_background_animator.cc b/ash/common/shelf/shelf_background_animator.cc
index 548b826884e195347ad7f1669eb0591bec5bc152..177473b6dec21a1a63618200916a1788f6609f70 100644
--- a/ash/common/shelf/shelf_background_animator.cc
+++ b/ash/common/shelf/shelf_background_animator.cc
@@ -98,14 +98,6 @@ void ShelfBackgroundAnimator::AnimationEnded(const gfx::Animation* animation) {
   animator_.reset();
 }
 
-void ShelfBackgroundAnimator::AnimationCanceled(

[msw, 2017/03/14 22:29:11]

I don't quite understand how this fixes the issue in the CL. Why is a shelf
background animation started and cancelled mid-drag, when the window never stops
overlapping the shelf?

What was the original purpose of this code? What would preempt this animation
such that we'd ever want to snap to the end state? We should make sure this
doesn't introduce another regression (ie. that this really isn't needed).

[bruthig, 2017/03/14 22:50:00]

You're right, there is another confounding factor here.  When a window is being
dragged it is moved from the kShellWindowId_DefaultContainer to the
kShellWindowId_DockedContainer (for reasons I'm not really sure, but I think it
has to do with z-order of the dragged window relative to the docked windows).

When this re-parenting happens ShelfLayoutManager::UpdateVisibilityState() is
called and WorkspaceController::GetWindowState() incorrectly reports that the
shelf is in WORKSPACE_WINDOW_STATE_DEFAULT mode because for a period of time the
dragged window is not in either container.

So the net effect was the background was starting to animate to the DEFAULT
state, getting preempted immediately and causing the shelf to snap to the
DEFAULT state before ending in the correct OVERLAP state.
This code never should have been included in the original change, it was an
oversight on my part.

A valid preemption will occur if a user quickly drags a window to overlap and
then to not-overlap we don't want the shelf to snap to the DEFAULT (aka
not-overlapped) state, we want it to smoothly animate back and forth.

-    const gfx::Animation* animation) {
-  DCHECK_EQ(animation, animator_.get());
-  SetAnimationValues(animator_->IsShowing() ? 1.0 : 0.0);
-  // Animations are only cancelled when they are being pre-empted so we don't
-  // destroy the |animator_| because it may be re-used immediately.
-}
-
 void ShelfBackgroundAnimator::OnWallpaperDataChanged() {}
 
 void ShelfBackgroundAnimator::OnWallpaperColorsChanged() {