Override hasPendingActivity() instead of using set/unsetPendingActivity() methods.

Asynchronous dropProtection is also removed since now events are not
dispatched inside stop().

BUG=none

Review URL: https://codereview.chromium.org/80473002

git-svn-id: svn://svn.chromium.org/blink/trunk@163064 bbb929c8-8fbe-4397-9dbb-9b2b20218538