Using ThreadTaskRunnerHandle in lieu of MessageLoopProxy  in content/browser/service_worker module

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in content/browser/service_worker module.

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

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

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045

Review URL: https://codereview.chromium.org/1129753002

Cr-Commit-Position: refs/heads/master@{#328709}
4 files changed