Fix marking up a page as prerendered with new TabContents-based approach.
Fix marking up a page as prerendered with new TabContents-based approach.

This makes the prerender icon reappear, as well as fixes issues with histograms.

BUG=None
TEST=Navigate to a prerendered page, see icon.


Review URL: http://codereview.chromium.org/7016019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@85237 0039d316-1c4b-4281-b951-d872f2087c98

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