Add SiteIsolation.IsolateExtensions metrics and tests.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index fe8fb358060b07f154868a01334a0a1eb8f62eb7..71312872069ae7f9477c902f65101c50f7b01624 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -170,6 +170,14 @@ void NotifyCacheOnIO(
 bool CollectSites(BrowserContext* context,
                   std::set<GURL>* sites,
                   FrameTreeNode* node) {
+  // Record about:blank as a real (process-having) site only if the SiteInstance
+  // is unassigned. Do not not otherwise depend on the siteinstance's site URL,

[nasko, 2015/09/04 22:19:43]

nit: double not, SiteInstance

[ncarter (slow), 2015/09/10 19:08:59]

Done.

+  // since it's value reflects the current process model, and this function

[nasko, 2015/09/04 22:19:43]

nit: its

[ncarter (slow), 2015/09/10 19:08:59]

Done.

+  // ought to behave identically across all process models.
+  if (node->current_url() == GURL(url::kAboutBlankURL) &&
+      node->current_frame_host()->GetSiteInstance()->HasSite()) {
+    return true;
+  }
   sites->insert(SiteInstance::GetSiteForURL(context, node->current_url()));
   return true;
 }