Add null check for |event_handler_| in DirectManipulationHelper dtor.

Root cause is that if |DirectManipulationHelper::Initialize| returns
false, then DirectManipulationHelper hasn't been properly initialized,
and it's member |event_handler_| may still be nullptr. In destructor
of DirectManipulationHelper, |DirectManipulationHelper::Destroy| is
called which references |event_handler_| without a null check result in
AV. Fix is to add null check before referencing |event_handler_|.

Bug: 1001325
Change-Id: I815583413cb3e4c545568ea4ccba3b29366dedce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857546Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Siye Liu <siliu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#705715}

content/browser/renderer_host/direct_manipulation_helper_win.cc

@@ -268,6 +268,8 @@ void DirectManipulationHelper::Destroy() {
   if (has_animation_observer_)
     RemoveAnimationObserver();
   compositor_ = nullptr;
+
+  if (event_handler_)
     event_handler_->SetDirectManipulationHelper(nullptr);
 
   HRESULT hr;