Remove CRX installation feature from Files.app.

This is to align the behavior with other platforms.
The only allowed way to install extensions from .crx is
by the drag and drop to the settings page.

BUG=308347
R=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/29563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230062 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230062