SyncFS: Add ConflictResolverTest


BUG=240165
TEST=unit_tests --gtest_filter=ConflictResolverTest.*
NOTRY=true

Review URL: https://codereview.chromium.org/98053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238307 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238307