Hide BrowsingInstance from all but SiteInstance, as intended by design.

content/browser/site_instance_unittest.cc

Index: content/browser/site_instance_unittest.cc
diff --git a/content/browser/site_instance_unittest.cc b/content/browser/site_instance_unittest.cc
index ebb532c353b5a3c024638685ced586484e1668a8..f436471162a6857fbfed408a70587a43632b4442 100644
--- a/content/browser/site_instance_unittest.cc
+++ b/content/browser/site_instance_unittest.cc
@@ -133,6 +133,14 @@ class TestBrowsingInstance : public BrowsingInstance {
     use_process_per_site_ = use_process_per_site;
   }
 
+  // Make a few methods public for tests.
+  using BrowsingInstance::ShouldUseProcessPerSite;
+  using BrowsingInstance::browser_context;
+  using BrowsingInstance::HasSiteInstance;
+  using BrowsingInstance::GetSiteInstanceForURL;
+  using BrowsingInstance::RegisterSiteInstance;
+  using BrowsingInstance::UnregisterSiteInstance;
+
  private:
   virtual ~TestBrowsingInstance() {
     (*delete_counter_)++;