LockGuard is mostly used with Mutex. Since both are defined outside the
internal namespace, we often have to write
{base::LockGuard<base::Mutex>}. This CL shortens this to
{base::MutexGuard} across the code base

R=mlippautz@chromium.org

Bug: v8:8238
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I020d5933b73aafb98c4b72e3bb2dfd07c979ba73
Reviewed-on: https://chromium-review.googlesource.com/c/1278796Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56612}