Implemented history for alerts. This doesn't include an endpoint to retrieve them yet.

BUG=416182
R=ojan@chromium.org
CC=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/596393002

git-svn-id: svn://svn.chromium.org/blink/trunk@183070 bbb929c8-8fbe-4397-9dbb-9b2b20218538