[SyncFS] Add bulk update method to MetadataDatabase

Adding a helper for folder create-create conflict handling.

BUG=240165
R=kinuko@chromium.org, nhiroki@chromium.org

Review URL: https://codereview.chromium.org/95113005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238894 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238894