[chromedriver] Modify chromedriver buildbots to test against latest Blink builds rather than Chromium builds.

ChromeDriver is unaffected by most browser-side changes, but Blink/DevTools changes occasionally cause test failures. By testing ChromeDriver against the latest Blink builds, we can catch these regressions earlier.

Also disable testShouldHandleNewWindowLoadingProperly for now, so that it doesn't hide other test failures.

BUG=chromedriver:815
NOTRY=true

Review URL: https://codereview.chromium.org/313423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275278 0039d316-1c4b-4281-b951-d872f2087c98
[chromedriver] Pass --test-type=webdriver when launching Chrome.

This prevents Chrome from creating an infobar warning about unsupported flags (in particular --ignore-certificate-errors, which we also pass when launching Chrome).

BUG=chromedriver:799

Review URL: https://codereview.chromium.org/313003005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275296 0039d316-1c4b-4281-b951-d872f2087c98