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 722230a99199d0d627ebdec0efd75839e726b3f9..afbf3db6f005640a998e8b0fc6c3b1648cd2ee67 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 otherwise depend on the siteinstance's site URL,
+  // since its value reflects the current process model, and this function
+  // should 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;
 }