Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in net/socket/ module.

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

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

BUG=391045

Review URL: https://codereview.chromium.org/1126553002

Cr-Commit-Position: refs/heads/master@{#328248}
1 file changed