There's potential a race condition between Finalize and
InitializeWithGUID, if the browser context decides to call Finalize
before GetOrCreateGUID has finished. But Finalize will never actually be
called anyway, nor is it needed, so remove it along with the unused
WebView versions of PostMainMessageLoopRun.

Also fix up threading comments and add some more threading DCHECKs.

BUG=546754

Review URL: https://codereview.chromium.org/1584483004

Cr-Commit-Position: refs/heads/master@{#370431}