Fix crash when closing page with a scroll event handler

This patches fixes a null dereference when the final scroll event
listener is removed from the EventHandlerRegistry during WebView
teardown. More specifically, if ScrollingCoordinator is told to update
its knowledge about scroll event listeners after the main frame has
become detached from its respective view, the coordinator will try
to deref a null pointer when looking for the root scrolling layer.

This patch adds a guard against a detached frame into
ScrollingCoordinator's scroll handler logic. The wheel and touch event
handler functions are also guarded as they will soon be triggered by the
same mechanism in EventHandlerRegistry.

TEST=ScrollingCoordinatorChromiumTest.updateEventHandlersDuringTeardown
BUG=371905,359566

Review URL: https://codereview.chromium.org/275563006

git-svn-id: svn://svn.chromium.org/blink/trunk@174099 bbb929c8-8fbe-4397-9dbb-9b2b20218538