OpenFileSystem with create=true whenever we need to initialize SyncFS for an origin

In SyncFS it's assumed that all filesystems are opened in the writable
mode, so that remote changes can be applied to the local files even for
the ones that haven't been initialized yet. (Or we could just hit assertion)

BUG=none
TEST=manual (make remote changes for the app that hasn't been opened
yet locally)

Review URL: https://codereview.chromium.org/112393005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240083 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240083