Add a private API method to install a webstore app from Files.app

This patch is a part of the Webstore-Files.app integration (crbug.com/240152).

BUG=272245
TEST=manually tested

Review URL: https://chromiumcodereview.appspot.com/23332012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@220581 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed