Move PlatformEvent.cpp/.h to Source/platform/

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25527004

git-svn-id: svn://svn.chromium.org/blink/trunk@158974 bbb929c8-8fbe-4397-9dbb-9b2b20218538