Fix ARC notification crash.

Fix ARC notification crash.

Make ArcCustomNotificationView::SlideHelper operate on the correct
layer, i.e. that which is sliding out.

This is meant as a temporary hack to address the crash, but the code
still needs to be restructured to reduce fragility and improve test
coverage.

BUG=718965
Review-Url: https://codereview.chromium.org/2863163002
Cr-Original-Commit-Position: refs/heads/master@{#469787}
Review-Url: https://codereview.chromium.org/2886283006 .
Cr-Commit-Position: refs/branch-heads/3071@{#632}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/8f75642cf2ecc597f87a8a66aa227ec79e947db1

[Evan Stade, 2017-05-19 15:22:23]

Description was changed from

==========
Fix ARC notification crash.

Make ArcCustomNotificationView::SlideHelper operate on the correct
layer, i.e. that which is sliding out.

This is meant as a temporary hack to address the crash, but the code
still needs to be restructured to reduce fragility and improve test
coverage.

BUG=718965

Review-Url: https://codereview.chromium.org/2863163002
Cr-Commit-Position: refs/heads/master@{#469787}
(cherry picked from commit 99c145d628465ca7c71f46ae39fe875466e3fb5e)
==========

to

==========
Fix ARC notification crash.

Make ArcCustomNotificationView::SlideHelper operate on the correct
layer, i.e. that which is sliding out.

This is meant as a temporary hack to address the crash, but the code
still needs to be restructured to reduce fragility and improve test
coverage.

BUG=718965

Review-Url: https://codereview.chromium.org/2863163002
Cr-Original-Commit-Position: refs/heads/master@{#469787}
Review-Url: https://codereview.chromium.org/2886283006 .
Cr-Commit-Position: refs/branch-heads/3071@{#632}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/8f75642cf2ecc597f87a8a66aa22...
==========

[Evan Stade, 2017-05-19 15:22:25]

Committed patchset #1 (id:1) manually as
8f75642cf2ecc597f87a8a66aa227ec79e947db1.