drive: Less compilicated ChangeListLoader with cached AboutResource

Merge heavily branched code paths around fast-fetch.
cached_about_resource_ is guaranteed to be non-NULL after LoadAfterGetAboutResource().

DoInitialLoad(), DoUpdateLoad() and LoadFromServerIfNeeded() are merged and renamed to LoadAfterGetLargestChangestamp().
LoadFromServerIfNeededAfterGetAbout() is renamed to LoadAfterGetAboutResource().
LoadFromServerIfNeededAfterLoadDirectory() is renamed to LoadAfterLoadDirectory().

BUG=260526
TEST=unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/103803013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241778 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241778
Add Sync datatype for shared managed user settings.

BUG=316168

Review URL: https://codereview.chromium.org/76333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241779 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241779