[SyncFS] Add conflict case tests to DriveBackendSynctest

 * Add conflict case tests
 * Fix missing MetadataDatabase update on local-to-remote sync

BUG=331988

Review URL: https://codereview.chromium.org/156023003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250178 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250178
Use validating IPC enum macros in child process messages.

This is part of a long-running background task to remove the remaining
uses of the unchecked IPC_ENUM_TRAITS() macro.

BUG=246708

Review URL: https://codereview.chromium.org/131453015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250181 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250181