Move GoogleAutoLoginHelper to //google_apis.

This CL does the following:
- Eliminates //chrome and //content dependences from GoogleAutoLoginHelper and
  its unittest.
- Moves GoogleAutoLoginHelper to //google_apis and renames it
  MergeSessionHelper.

The specific motivation for the code movement is to enable iOS to cleanly use
this code. The renaming is for clarity.

BUG=330292
TBR=nyquist

Review URL: https://codereview.chromium.org/135903011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246390 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246390