Do not show menu items added by extensions inside <webview>.

Do not show menu items added by extensions inside <webview>.
<webview>'s are supposed to be isolated from regular tab/pages, so
context menus from other extensions should not appear inside them.

BUG=313907
Test=Install some extension that can add a context menu item to some
page, now create an app with <webview> in it and navigate the
<webview> to that page that shows the context menu in regular tab.
Notice that the added context menu item does not appear anymore
inside the <webview>.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=232556

[lazyboy, 2013-11-01 18:41:59]

+avi for review.

[Avi (use Gerrit), 2013-11-01 18:56:16]

lgtm

[commit-bot: I haz the power, 2013-11-01 19:36:43]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/lazyboy@chromium.org/54973005/1

[commit-bot: I haz the power, 2013-11-01 22:32:32]

Retried try job too often on win_rel for step(s) content_unittests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2013-11-01 22:56:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/lazyboy@chromium.org/54973005/1

[commit-bot: I haz the power, 2013-11-02 02:04:08]

Change committed as 232556