Add UI test for Dromaeo benchmark. That required minor modification of Dromaeo benchmark to enable automation. BUG=none TEST=none Committing for Anton Muhin (antonm@chromium.org). Review URL: http://codereview.chromium.org/274020 Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 19783666440e33def6a060a558a661be245d8144