chrome: Support client-created request contexts (see issue #2969)

RequestContextTest and URLRequestTest suites now pass with the Chrome runtime
enabled.

libcef/browser/request_context_impl.h

@@ -128,6 +128,7 @@ class CefRequestContextImpl : public CefRequestContext {
   explicit CefRequestContextImpl(const Config& config);
 
   void Initialize();
+  void BrowserContextInitialized();
 
   // Make sure the browser context exists. Only called on the UI thread.
   void EnsureBrowserContext();