Don't swap processes for chrome:// subframes.

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 1dc50e79c0d7f1b3b0a266f8493823f1367d18e1..f22b83157efcc87a607166da383f4949366dbcf1 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1320,6 +1320,22 @@ RenderFrameHostManager::DetermineSiteInstanceForURL(
     return SiteInstanceDescriptor(current_instance_impl);
   }
 
+  // In the site-per-process model, process changes are skipped for chrome://
+  // subframes for now, since e.g. chrome://settings has multiple "cross-site"

[Charlie Reis, 2017/01/20 23:32:11]

Let's rephrase this a bit to more closely match what we're doing, since the code
isn't limited to --site-per-process, and simply skipping a swap for a chrome://
subframe isn't sufficient (e.g., it would be bad if the subframe started on the
web and then somehow went to chrome://).  Something like:

If the parent frame is a chrome:// page and the subframe is as well, keep the
subframe in the parent's process even if they would be considered different
sites.  This avoids unnecessary OOPIFs on pages like chrome://settings, which
currently has multiple "cross-site" subframes that don't need isolation.

[nasko, 2017/01/20 23:51:58]

Done.

+  // chrome:// frames, and that doesn't yet work cross-process.
+  if (SiteIsolationPolicy::AreCrossProcessFramesPossible() &&
+      !frame_tree_node_->IsMainFrame()) {
+    GURL parent_site_url = frame_tree_node_->parent()

[Charlie Reis, 2017/01/20 23:32:11]

nit: const ref

[nasko, 2017/01/20 23:51:57]

No longer present.

+                               ->current_frame_host()
+                               ->GetSiteInstance()

[Charlie Reis, 2017/01/20 23:32:11]

Let's define parent_site_instance as a local variable to make this and the line
below more legible.

[nasko, 2017/01/20 23:51:57]

Done.

+                               ->GetSiteURL();
+    if (parent_site_url.SchemeIs(kChromeUIScheme) &&
+        dest_url.SchemeIs(kChromeUIScheme)) {
+      return SiteInstanceDescriptor(
+          frame_tree_node_->parent()->current_frame_host()->GetSiteInstance());
+    }
+  }
+
   // If we haven't used our SiteInstance (and thus RVH) yet, then we can use it
   // for this entry.  We won't commit the SiteInstance to this site until the
   // navigation commits (in DidNavigate), unless the navigation entry was