Fix two URLRequestJob subclasses that didn't support cancellation

This was causing crashes.  I suspect there are a bunch of other
subclasses with the same issue.  :(

Also add an OWNERS file to components/about_handler.

BUG=508900,541152

Review URL: https://codereview.chromium.org/1395643003

Cr-Commit-Position: refs/heads/master@{#353271}
5 files changed