fetch_client: Support file URL scheme

This is done by moving to URLFetcher rather than HttpNetworkTransaction. This both simplifies the code for fetch_client, and makes it closer to a typical consumer of the networking library.

This CL also fixes URLRequestContextBuilder::DisableHttpCache, which I discovered was broken while making this change.

BUG=335585

Review URL: https://codereview.chromium.org/134603004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245803 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245803