Rename SameThreadClosure to Closure

BUG=624668
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2110363002
Cr-Commit-Position: refs/heads/master@{#403287}
25 files changed