Don't create unexpected OOPIFs in --isolate-extensions.

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 dab752ccce8ef6062353966fc765bb3091c2e4f1..3bc5b185b5dd34bce27e42db390a9059c86d7e55 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -2319,11 +2319,22 @@ RenderFrameHostImpl* RenderFrameHostManager::UpdateStateForNavigate(
     bool dest_is_view_source_mode,
     const GlobalRequestID& transferred_request_id,
     int bindings) {
-  // Don't swap for subframes unless we are in --site-per-process.  We can get
-  // here in tests for subframes (e.g., NavigateFrameToURL).
-  if (!frame_tree_node_->IsMainFrame() &&
-      !SiteIsolationPolicy::AreCrossProcessFramesPossible()) {
-    return render_frame_host_.get();
+  if (!frame_tree_node_->IsMainFrame()) {
+    // Don't swap for subframes unless we are in an OOPIF-enabled mode.  We can
+    // get here in tests for subframes (e.g., NavigateFrameToURL).
+    if (!SiteIsolationPolicy::AreCrossProcessFramesPossible())

[ncarter (slow), 2015/12/08 20:14:13]

I think it might be cleaner to structure this as follows:

if (!frame_tree_node_->IsMainFrame()) {
  if (!IsRendererTransferNeededForNavigation(render_frame_host_.get(),
                                             dest_url)) {
    return render_frame_host_.get();
  }
  // Continuing in this case will result in an out-of-process iframe.
  CHECK(SiteIsolationPolicy::AreCrossProcessFramesPossible());
}

[Charlie Reis, 2015/12/08 20:34:11]

Sure.  As you noted offline, we may not get to the code below in
--site-per-process if we're already in the right site, but that's basically a
no-op anyway.  (We'll also need to improve
IsRendererTransferNeededForNavigation, but we already knew that.)

+      return render_frame_host_.get();
+
+    // If we are in an OOPIF mode that only applies to some sites, only swap if
+    // the policy determines that a transfer would have been needed.  We can get
+    // here for session restore.
+    if (!SiteIsolationPolicy::UseDedicatedProcessesForAllSites() &&

[ncarter (slow), 2015/12/08 20:14:13]

I don't think you need to check UseDedicatedProcessesForAllSites here.
IsRendererTransferNeededForNavigation takes it into consideration.

[Charlie Reis, 2015/12/08 20:34:11]

Acknowledged.

+        !IsRendererTransferNeededForNavigation(render_frame_host_.get(),
+                                               dest_url)) {
+      DCHECK(!dest_instance ||
+             dest_instance == render_frame_host_->GetSiteInstance());
+      return render_frame_host_.get();
+    }
   }
 
   SiteInstance* current_instance = render_frame_host_->GetSiteInstance();