Reland "mac: Layer back the root view of the browser window on OSX 10.9+."

Reland "mac: Layer back the root view of the browser window on OSX 10.9+."

The original CL was reverted because it caused non-deterministic failures in
browser tests that used the file tab-restore-visibility.html. The test file had
an edge case that caused thrashing and prevented the page from loading. This
has since been fixed.

> BrowserWindowController always layer-backs the contentView of the window. If
> Chrome were linked against OSX 10.9, this would also cause the root view to be
> layer backed. Since Chrome is linked against OSX 10.6, the root view by default
> is not layer backed.
>
> BrowserWindowController is going to have a custom AppKit fullscreen animation,
> which requires that the root view is layer backed. While it is possible to
> implement a custom AppKit fullscreen animation as early as OSX 10.7, the
> effects of layer backing the root view on OSX 10.7 are murky. It seems much
> safer to just layer back the root view on OSX 10.9+, and only enable the custom
> AppKit fullscreen animation on OSX 10.9+.
>
> As far as I know, this change has no detrimental side effects. There is no
> spike in memory usage
> (https://code.google.com/p/chromium/issues/detail?id=396264#c11), presumably
> because NSThemeFrame implements -wantsUpdateLayer on OSX 10.9+.
>
> BUG=413009, 414527
> Committed: https://crrev.com/8dec34f8f3504352e4eef95b2aaa36c561aea930
> Cr-Commit-Position: refs/heads/master@{#311191}

BUG=413009, 414527
TBR=rsesek@chromium.org

Committed: https://crrev.com/4713180802457f8c6854c10db4689ab4fab98155
Cr-Commit-Position: refs/heads/master@{#313207}

[erikchen, 2015-01-27 01:38:51]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-01-27 01:39:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/878673006/1

[commit-bot: I haz the power, 2015-01-27 02:29:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 02:31:06]

Patchset 1 (id:??) landed as
https://crrev.com/4713180802457f8c6854c10db4689ab4fab98155
Cr-Commit-Position: refs/heads/master@{#313207}

[erikchen, 2015-01-27 23:35:42]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/881133002/ by erikchen@chromium.org.

The reason for reverting is: Chrome Mac lost its rounded bottom corners
https://code.google.com/p/chromium/issues/detail?id=452707.