[Mac] Improve modal sheet positioning during window resizing.

[Mac] Improve modal sheet positioning during window resizing.

Change sheet repositioning to be based on NSWindowDidResizeNotification,
because it tracks window resizing much better than
NSViewFrameDidChangeNotification.
The modal sheet should now track the parent window's resizing much more
smoothly.

BUG=358627
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=272421

[Andre, 2014-05-21 16:54:25]

Nico or Avi, can one of you review this change?

[Avi (use Gerrit), 2014-05-21 17:47:42]

lgtm

[Andre, 2014-05-21 17:48:13]

The CQ bit was checked by andresantoso@chromium.org

[commit-bot: I haz the power, 2014-05-21 19:58:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/andresantoso@chromium.org/299573002/1

[commit-bot: I haz the power, 2014-05-22 07:08:25]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-22 07:59:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-22 07:59:06]

Try jobs failed on following builders:
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[Andre, 2014-05-22 19:27:29]

The CQ bit was checked by andresantoso@chromium.org

[commit-bot: I haz the power, 2014-05-22 19:28:40]

CQ is trying da patch. Follow status at

https://chromium-status.appspot.com/cq/andresantoso@chromium.org/299573002/20001

[commit-bot: I haz the power, 2014-05-22 23:17:14]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-23 07:06:52]

Change committed as 272421