Docserver: Further refactoring to the new GithubFileSystem to make it update
content properly. Change it to behave the same way on Read and Refresh (in fact
Refresh becomes just an alias for Read) to address the bug where having
concurrent Read and Refresh futures overwrite each other.

BUG=318878
R=yoz@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/82433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236877 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236877