Fix crash in Optional.value()

This was introduced in https://crrev.com/c/2142514 when the invalid
value being case to an EventDisposition enum was turned into a
base::Optional de-ref. This situation should never occur but the crashes
indicate it does.

This CL preverves old behavior to prevent the crash.

TBR=nzolghadr@chromium.org

Bug: 1069760
Change-Id: I412edf2bff202d68730a3374792f1dab16a91426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145567
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758268}

ui/events/blink/input_handler_proxy.cc

@@ -809,10 +809,13 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
     // Noncancellable wheel events should have phase info.
     DCHECK(wheel_event.phase != WebMouseWheelEvent::kPhaseNone ||
            wheel_event.momentum_phase != WebMouseWheelEvent::kPhaseNone);
-    DCHECK(mouse_wheel_result_.has_value());
 
+    DCHECK(mouse_wheel_result_.has_value());
+    // TODO(bokan): This should never happen but after changing
+    // mouse_event_result_ to a base::Optional, crashes indicate that it does
+    // so |if| maintains prior behavior. https://crbug.com/1069760.
+    if (mouse_wheel_result_.has_value()) {
       result = mouse_wheel_result_.value();
-
       if (wheel_event.phase == WebMouseWheelEvent::kPhaseEnded ||
           wheel_event.phase == WebMouseWheelEvent::kPhaseCancelled ||
           wheel_event.momentum_phase == WebMouseWheelEvent::kPhaseEnded ||
@@ -822,6 +825,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
         return result;
       }
     }
+  }
 
   gfx::PointF position_in_widget = wheel_event.PositionInWidget();
   if (input_handler_->HasBlockingWheelEventHandlerAt(