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Page content often isn't completely loaded or laid out when
the tab is first rendered.  This makes the class
additionally notify about subsequent renders so the delegate
can hold off until it sees one after it's e.g. received
notification via a WebUI message that the page is truly
ready.

BUG=chromium-os:26210
TEST=manual; also un-disabled the tests and checked that they still pass
TBR=ben@chromium.org

Review URL: http://codereview.chromium.org/9582012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124765 0039d316-1c4b-4281-b951-d872f2087c98