Ephemeral apps initial UI changes for app launcher

This initial batch of UI changes includes:
- Added "Launch" button to WebStore results in the app launcher,
intended to launch apps without installing in Chrome. However, the app
currently will be installed and launched.
- If the app does not have any permission warnings, the launch prompt
will be skipped.

This functionality is hidden behind the "enable-ephemeral-apps" switch.

BUG=None

Review URL: https://codereview.chromium.org/35543005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231272 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231272