[Media Controls] Fix overlay play button crash for legacy controls

This CL adds a ModernMediaControls check before checking whether an
event is over the internal overlay play button, since the internal
button only exists on modern controls. This fixes a crash in the legacy
controls which dereferenced this null internal button.

Bug: 870490
Change-Id: Ic0990ffd1f4eb5c1f576c38191eb27848eedd9c9
Reviewed-on: https://chromium-review.googlesource.com/1163284Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581083}

third_party/blink/renderer/modules/media_controls/elements/media_control_overlay_play_button_element.cc

@@ -273,7 +273,7 @@ bool MediaControlOverlayPlayButtonElement::KeepEventInNode(Event* event) {
     return false;
 
   // For mouse events, only keep in node if they're on the internal button.
-  if (event->IsMouseEvent())
+  if (event->IsMouseEvent() && MediaControlsImpl::IsModern())
     return IsMouseEventOnInternalButton(ToMouseEvent(event));
 
   return true;