Files.app: Fix exceptions in the gallery.

The CL fixes the following two errors.

* The toURL method of an entry variable is called though the variable can be null when we select multiple items.
* Passes entries to the executeTask private API, but the API takes URLs.

BUG=340552
TEST=execute the code path.

Review URL: https://codereview.chromium.org/155593003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249027 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249027