[SyncFS] Add integration tests of the drive backend

 * Add drive_backend level integration test class
 * Add basic test case

BUG=331988
TEST=./unit_tests --gtest_filter='DriveBackendSyncTest.*'

Review URL: https://codereview.chromium.org/97703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243280