Move EventRouter to src/extensions/browser

Now that its dependencies on Profile, ActivityLog, RuntimeAPI and
WebRequestAPI have been removed it can move into the extensions module.

This is part of the AppShell extensions refactoring project.

BUG=316367
TEST=compiles
R=yoz@chromium.org
TBR=mukai@chromium.org for mechanical file move in c/b/notifications, dmazzoni@chromium.org for mechanical file move in accessibility code

Review URL: https://codereview.chromium.org/78483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236549 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236549