Allow SiteInstance for about:blank in new tab to be reused.
Also fix flaky tests from the resulting speedup.

BUG=111975
TEST=Open a new tab to about:blank. No process swap for nav to Google.

Review URL: http://codereview.chromium.org/9264049

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@120621 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed