Fix crash in event sender

Event sender is used only in web_tests. Tests shouldn't ever set this
case up but we shouldn't ever crash.

Bug: 996071
Change-Id: I4fa1ef12d934b1698ca2f70070bcb509355dd491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763375Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689094}

content/shell/test_runner/event_sender.cc

@@ -1379,6 +1379,9 @@ int EventSender::ModifiersForPointer(int pointer_id) {
 
 void EventSender::DoDragDrop(const WebDragData& drag_data,
                              WebDragOperationsMask mask) {
+  if (!mainFrameWidget())
+    return;
+
   WebMouseEvent raw_event(WebInputEvent::kMouseDown,
                           ModifiersForPointer(kRawMousePointerId),
                           GetCurrentEventTime());
@@ -2926,6 +2929,8 @@ blink::WebFrameWidget* EventSender::mainFrameWidget() {
   DCHECK(view()->MainFrame()->IsWebLocalFrame())
       << "Event Sender doesn't support being run in a remote frame for this "
          "operation.";
+  if (!view() || !view()->MainFrame())
+    return nullptr;
   return view()->MainFrame()->ToWebLocalFrame()->FrameWidget();
 }