Allow app_shell embedders to override extensions client interfaces

extensions/shell/browser/shell_browser_main_parts.cc

Index: extensions/shell/browser/shell_browser_main_parts.cc
diff --git a/extensions/shell/browser/shell_browser_main_parts.cc b/extensions/shell/browser/shell_browser_main_parts.cc
index a454942a8772b2eb7ba1b0f30ed972e26a97371c..bf435d3d2fd3234b4e3459db7abecbec26bfb1a0 100644
--- a/extensions/shell/browser/shell_browser_main_parts.cc
+++ b/extensions/shell/browser/shell_browser_main_parts.cc
@@ -139,11 +139,11 @@ void ShellBrowserMainParts::PreMainMessageLoopRun() {
 
   device_client_.reset(new ShellDeviceClient);
 
-  extensions_client_.reset(new ShellExtensionsClient());
+  extensions_client_.reset(CreateExtensionsClient());
   ExtensionsClient::Set(extensions_client_.get());
 
   extensions_browser_client_.reset(
-      new ShellExtensionsBrowserClient(browser_context_.get()));
+      CreateExtensionsBrowserClient(browser_context_.get()));
   ExtensionsBrowserClient::Set(extensions_browser_client_.get());
 
   omaha_query_params_delegate_.reset(new ShellOmahaQueryParamsDelegate);
@@ -248,6 +248,15 @@ void ShellBrowserMainParts::PostDestroyThreads() {
 #endif
 }
 
+ExtensionsClient* ShellBrowserMainParts::CreateExtensionsClient() {
+  return new ShellExtensionsClient();
+}
+
+ExtensionsBrowserClient* ShellBrowserMainParts::CreateExtensionsBrowserClient(
+    content::BrowserContext* context) {
+  return new ShellExtensionsBrowserClient(context);
+}
+
 void ShellBrowserMainParts::CreateExtensionSystem() {
   DCHECK(browser_context_);
   extension_system_ = static_cast<ShellExtensionSystem*>(