App Launcher on Linux is now positioned to the shelf/mouse cursor.

Currently, this is tailored for Unity, assuming the shelf is on the left
edge. In subsequent patches, we will add functionality for different
desktop environments.

BUG=299250

Review URL: https://codereview.chromium.org/79433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239975 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239975