Remove NOTREACHED() in OutOfProcessInstance::SendInputEventToEngine().

It is apparently reachable, since ConvertEventTypes() in
content/renderer/pepper/event_conversion.cc can create input events of
type `PP_INPUTEVENT_TYPE_UNDEFINED` for a variety of reasons, and PPAPI
does not always send `PP_INPUTEVENT_TYPE_CONTEXTMENU` events through
correctly.

Bug: 1124478
Change-Id: Id376f948a78ca9d33ee40e825d7f131a11f61ec1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2392376Reviewed-by: K. Moon <kmoon@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804436}

pdf/out_of_process_instance.cc

@@ -2287,7 +2287,6 @@ bool OutOfProcessInstance::SendInputEventToEngine(const pp::InputEvent& event) {
       // instead.
       return engine()->HandleEvent(NoneInputEvent());
     case PP_INPUTEVENT_TYPE_UNDEFINED:
-      NOTREACHED();
       return false;
   }
 }