Initialize GPU shader disk cache on brower thread.

content/public/browser/content_browser_client.h

Index: content/public/browser/content_browser_client.h
diff --git a/content/public/browser/content_browser_client.h b/content/public/browser/content_browser_client.h
index b9c0f419c5d3e2528301a7f8c637c8aa93bfb551..2af6ccb748b5038099996c8d2fdc17a1d8788a1a 100644
--- a/content/public/browser/content_browser_client.h
+++ b/content/public/browser/content_browser_client.h
@@ -514,6 +514,9 @@ class CONTENT_EXPORT ContentBrowserClient {
   // else we should do with the file.
   virtual std::string GetDefaultDownloadName();
 
+  // Returns the path to the user data directory.
+  virtual base::FilePath GetUserDataDirectory();
+
   // Notification that a pepper plugin has just been spawned. This allows the
   // embedder to add filters onto the host to implement interfaces.
   // This is called on the IO thread.