Migrate from fullPath to Entry in the mosaic view.

This patch removes the last instance of fullPath in the mosaic view and replaces it with use of the volume manager to extract the location information.

TEST=Tested manually.
BUG=320967

Review URL: https://codereview.chromium.org/130383003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244126 0039d316-1c4b-4281-b951-d872f2087c98
Point out in the comments the equivalence of two observer functions.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/132063006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244127 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244127