Extract OAuth2AccessTokenFetcher interface.

This CL extracts the interface of OAuth2AccessTokenFetcher as the 
implementation for mobile is different .
It renames the existing OAuth2AccessTokenFetcher to 
OAuth2AccessTokenFetcherImpl.

TBR=davemoore@chromium.org
BUG=NONE

Review URL: https://codereview.chromium.org/182573003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254966 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254966