Move load credentials to MutableProfileOAuth2TokenService

This CL moves the web_data_service_request_ field from
ProfileOAuth2TokenService to MutableProfileOAuth2TokenService.

TEST=NONE (no user visible change)
BUG=320625

Review URL: https://codereview.chromium.org/100033004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239801