Reland r165040 "Make ancestors of modal <dialog> inert"

Reland r165040 "Make ancestors of modal <dialog> inert"

Original code review: https://codereview.chromium.org/133393002/
r165040 was reverted due to failing a11y tests in
content_browsertests; they have been disabled and will be rebaselined
after this change is rolled in.

BUG=329407
TBR=esprehn

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165211

[commit-bot: I haz the power, 2014-01-16 04:47:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/139183002/1

[commit-bot: I haz the power, 2014-01-16 07:20:41]

Retried try job too often on win_blink_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_blink_...

[commit-bot: I haz the power, 2014-01-16 08:58:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/139183002/1

[commit-bot: I haz the power, 2014-01-16 11:07:40]

Retried try job too often on win_blink_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_blink_...

[commit-bot: I haz the power, 2014-01-16 11:08:52]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/139183002/1

[commit-bot: I haz the power, 2014-01-16 11:43:34]

Change committed as 165211