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This class will be used to handle prerender requests coming from external applications.
For better tracking and also supportig referrer, a separate PrerenderManager API is
added.

BUG=297859

Review URL: https://codereview.chromium.org/45693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233457 0039d316-1c4b-4281-b951-d872f2087c98