Turn on AreCrossProcessFramesPossible by default.

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index 2b9f1e3ed1be83b5c73eeeaa12f313ad59e12d4d..8dbeb4204cd50fe9f20b14afcc9b2524f2e2c7c8 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -355,6 +355,7 @@ void RenderWidgetHostViewGuest::UpdateCursor(const WebCursor& cursor) {
   // and so we will always hit this code path.
   if (!guest_)
     return;
+  // TODO(creis): This is wrong.  Should check for OOPIF webview mode instead.

[alexmos, 2017/02/04 02:03:01]

I actually think this is ok.  The description in the CL that introduced this,
https://codereview.chromium.org/1845083002, seems to imply that this should
trigger based on whether the input event router is installed, which James
confirmed should be triggered by AreCrossProcessFramesPossible().  I chatted
with Lucas, and he also agreed that this call is ok as is.

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

Acknowledged.

   if (SiteIsolationPolicy::AreCrossProcessFramesPossible()) {
     RenderWidgetHostViewBase* rwhvb = GetOwnerRenderWidgetHostView();
     if (rwhvb)