Add a non-blocking "OneShotEvent" class

to simplify code that needs to run after something has happened, and
start using it for the ExtensionService's READY notification.

This change doesn't, in itself replace any uses of NOTIFICATION_EXTENSIONS_READY, but it paves the way for doing so.

BUG=240968
R=atwilson@chromium.org, kalman@chromium.org, mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/14757022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@200918 0039d316-1c4b-4281-b951-d872f2087c98