Expose Timer::AbandonAndStop, so it can safely be destroyed on another thread.

BUG=chromium:736268
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/2958003002
Cr-Commit-Position: refs/heads/master@{#484507}
2 files changed