Prepare to 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..6de0f24d8ae81b9428617f5287e1fd7fc76a9562 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1098,6 +1098,9 @@ 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, alexmos): This looks like it will break single-process and
+  // process-per-tab.  See https://crbug.com/688617.
   if (SiteIsolationPolicy::AreCrossProcessFramesPossible())
     return true;