Pass "shared" metadata of Drive entries in fileBrowserPrivate API.

This information is necessary for distinguishing shared folders in Files.app UI.
The similarly named "shared_with_me" field has a different meaning that is also
necessary, hence we need to pass both. (The term and the semantics is derived
from Drive API; we're keeping it as-is for consistency. For the detail of the
difference, see comments in c/b/cros/drive/drive.proto.)

BUG=264812

Review URL: https://codereview.chromium.org/103883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238856 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238856