Wrapping up finch roll-out by making this default and switching to a
10% finch hold-back group in Canary.

R=vmiura@chromium.org
BUG=643850
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2844483003
Cr-Commit-Position: refs/heads/master@{#467198}