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This brings browser_unittest's run time from 21s to 7s on a Z620 Linux, one of our fastest
configurations. (We still need 3 browser launches to test browser options and profile loading.)

Add BrowserTestCase as the new parent class of TabTestCase. The separation is very clean :)
Also add helper method UrlOfUnittestFile - many tests are using http_server.UrlOf incorrectly,
and this helper ensures the HTTP server is running and we're pointing to the right file.
Also move page_test_test_case from testing/ to unittest/ and remove testing/.


BUG=388256
TEST=tools/telemetry/run_tests

Review URL: https://codereview.chromium.org/469593002

Cr-Commit-Position: refs/heads/master@{#289247}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289247 0039d316-1c4b-4281-b951-d872f2087c98