GitLab

Convert the last ramining instances of MessagePumpObservers into
PlatformEventObservers, and remove support for MessagePumpObservers
from non-Windows platforms.

BUG=354062
R=sky@chromium.org
TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/233653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263158 0039d316-1c4b-4281-b951-d872f2087c98