ash::ShelfView: CHECK against potentially bad static_casts.

ash::ShelfView: CHECK against potentially bad static_casts.

There are a number of static downcasts in ShelfView that are *probably*
correct, but there is no clear proof that they are casting to the
correct type. This patch adds explicit CHECKs before every static_cast
to ensure that we are not casting from an incompatible dynamic type.

drag_view_ now has type ShelfButton*, so it is downcast on assignment,
rather than on many of its uses.

BUG=418460
Committed: https://crrev.com/1373851514d52a205024caec9f26cc61a94f0fc2
Cr-Commit-Position: refs/heads/master@{#297335}

[Matt Giuca, 2014-09-29 07:12:37]

mgiuca@chromium.org changed reviewers:
+ skuhne@chromium.org, sky@chromium.org

[Matt Giuca, 2014-09-29 07:12:38]

Hi, can you both look at this? (sky wrote the original code with the ViewModel;
skuhne wrote the code with the drag_view_).

As I said in the bug, ideally we could avoid these static_casts by combining the
ShelfButton and AppListButton into a common type, but I think this is a good
first step.

[sky, 2014-09-29 17:51:26]

LGTM

[Mr4D (OOO till 08-26), 2014-09-29 18:05:47]

lgtm

[Matt Giuca, 2014-09-30 00:37:11]

The CQ bit was checked by mgiuca@chromium.org

[commit-bot: I haz the power, 2014-09-30 00:38:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/614653002/1

[commit-bot: I haz the power, 2014-09-30 01:44:48]

Committed patchset #1 (id:1) as 78335c0beee249831a75428be4586fa1c65caa4a

[commit-bot: I haz the power, 2014-09-30 01:45:51]

Patchset 1 (id:??) landed as
https://crrev.com/1373851514d52a205024caec9f26cc61a94f0fc2
Cr-Commit-Position: refs/heads/master@{#297335}