GitLab

This cleans up the browser tests in a number of ways:
- Add some convenience functions.
- Implement title-waiting outside of TestPrerenderContents.
- Remove most uses of GetPrerenderContents() in preparation for tests which
  manage two prerenders.
- Remove half of PrerenderTestURL variants by moving the one-off variations to
  the tests themselves.
- Move some initialization logic out of PrerenderTestURLImpl.

BUG=307592
TEST=Prerender*.*

Review URL: https://codereview.chromium.org/136583007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246128 0039d316-1c4b-4281-b951-d872f2087c98