Reposition notifications when the primary display changes as a result of unplugging a monitor on Linux Aura. 

BUG=267199
TEST=See Comment #5 in bug

Review URL: https://codereview.chromium.org/186413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254992