Add mount option parameter to ExternalMountPoints::RegisterFileSystem.

In order to fix the bug 276276, I'd like to vary the behavior
of copy among mount points. This CL is the preparation for
setting up the flag on each mount point.

BUG=276276
R=kinuko,benwells,satorux
TBR=sky

Review URL: https://codereview.chromium.org/99613004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240104 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240104