Turn on AreCrossProcessFramesPossible by default.

content/browser/renderer_host/render_widget_host_view_event_handler.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_view_event_handler.h"
 
 #include "base/metrics/user_metrics_action.h"
 #include "content/browser/renderer_host/input/touch_selection_controller_client_aura.h"
 #include "content/browser/renderer_host/overscroll_controller.h"
 #include "content/browser/renderer_host/render_view_host_delegate.h"
@@ skipping 822 matching lines @@
   //    one or more BrowserPlugins are present, then the event may be targeted
   //    to one of them, or this view. This allows GuestViews to have access to
   //    them while still forcing pinch-zoom to be handled by the top-level
   //    frame. TODO(wjmaclean): At present, this doesn't work for OOPIF, but
   //    it should be a simple extension to modify RenderWidgetHostViewChildFrame
   //    in a similar manner to RenderWidgetHostViewGuest.
   bool result = host_->delegate() && host_->delegate()->GetInputEventRouter() &&
                 !disable_input_event_router_for_testing_;
   // ScrollEvents get transformed into MouseWheel events, and so are treated
   // the same as mouse events for routing purposes.
+  // TODO(creis): Is this right?

[wjmaclean, 2017/02/03 17:01:53]

Yes, I think this is correct. (Although these days I think the mouse-wheels get
converted to GestureScrolls, and not the other way around).

We should start to look at the cases where there is no delegate, or the delegate
has no input event router, and see if they still exist.

There's already a bug filed to make |disable_input_event_router_for_testing_|
unnecessary.

[Charlie Reis, 2017/02/13 22:02:49]

Acknowledged.

   if (event->IsMouseEvent() || event->type() == ui::ET_SCROLL)
     result = result && SiteIsolationPolicy::AreCrossProcessFramesPossible();
   return result;
 }
 
 void RenderWidgetHostViewEventHandler::ProcessMouseEvent(
     const blink::WebMouseEvent& event,
     const ui::LatencyInfo& latency) {
   host_->ForwardMouseEventWithLatencyInfo(event, latency);
 }
 
 void RenderWidgetHostViewEventHandler::ProcessMouseWheelEvent(
     const blink::WebMouseWheelEvent& event,
     const ui::LatencyInfo& latency) {
   host_->ForwardWheelEventWithLatencyInfo(event, latency);
 }
 
 void RenderWidgetHostViewEventHandler::ProcessTouchEvent(
     const blink::WebTouchEvent& event,
     const ui::LatencyInfo& latency) {
   host_->ForwardTouchEventWithLatencyInfo(event, latency);
 }
 
 }  // namespace content