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The tests have changed significantly since they were first disabled. Add a
WaitForStop or two for good measure, but leave them as-is for the most part.
They can be disabled again if they still flake. Merge their custom prerender
loader into the main one; it's mostly the same.

In addition, for better test coverage, add a new test which asserts on events
received when a <link rel=prerender> is added for an existing prerender after
that prerender has loaded. Significantly rework the prerender events logic
to allow the test framework to wait on an event being received in the loader.

BUG=167340, 128841

Review URL: https://codereview.chromium.org/142013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251495 0039d316-1c4b-4281-b951-d872f2087c98