Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in chrome browser extensions module.

MessageLoopProxy is deprecated.
This basically does a search and replace:

MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().

BUG=391045

Review URL: https://codereview.chromium.org/1112573002

Cr-Commit-Position: refs/heads/master@{#328118}
24 files changed