Replace PluginThreadTaskRunner with a simpler MainPluginThreadTaskRunner.

PluginThreadTaskRunner was designed to work with NPAPI and PPAPI, but
it's no longer necessary. Replacing it with MainPluginThreadTaskRunner
that's Pepper-specific and is much simpler.

BUG=544275

Review URL: https://codereview.chromium.org/1410923004

Cr-Commit-Position: refs/heads/master@{#355200}
9 files changed