Turn on AreCrossProcessFramesPossible by default.

content/browser/frame_host/render_frame_host_manager.cc

Index: content/browser/frame_host/render_frame_host_manager.cc
diff --git a/content/browser/frame_host/render_frame_host_manager.cc b/content/browser/frame_host/render_frame_host_manager.cc
index ebba6861f7850377f4ddefd0b5755bc37631382e..e27ba94aa6a475c63bbbb074fd4f7bb01b558336 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1098,6 +1098,8 @@ bool RenderFrameHostManager::ShouldTransitionCrossSite() {
   // TODO(nick): Move all handling of kSingleProcess/kProcessPerTab into
   // SiteIsolationPolicy so we have a consistent behavior around the interaction
   // of the process model flags.
+  // TODO(creis): This looks like it will break --single-process and
+  // --process-per-tab.

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

Filed https://bugs.chromium.org/p/chromium/issues/detail?id=688617.  It does
appear that we are currently breaking cross-process navigations under
--single-process if we are also in an OOPIF mode.

   if (SiteIsolationPolicy::AreCrossProcessFramesPossible())
     return true;