Ensure static BrowserContext methods only get called on the UI thread.

content/public/browser/browser_context.h

Index: content/public/browser/browser_context.h
===================================================================
--- content/public/browser/browser_context.h	(revision 145676)
+++ content/public/browser/browser_context.h	(working copy)
@@ -44,7 +44,8 @@
 class SpeechRecognitionPreferences;
 
 // This class holds the context needed for a browsing session.
-// It lives on the UI thread.
+// It lives on the UI thread. All these methods must only be called on the UI
+// thread.
 class CONTENT_EXPORT BrowserContext : public base::SupportsUserData {
  public:
   static DownloadManager* GetDownloadManager(BrowserContext* browser_context);