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..e0ea6b2e62bb44ee6c18c2a6812ee38e9c45fa7a 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 browser shader disk cache root.

[jam, 2015/08/07 20:41:04]

nit: please make chrome return a path that includes the shader directory (i.e.
not root = udd). i would like to avoid sending udd directly to content module to
avoid layering violations with content mucking with chrome's files

[Jamie Madill, 2015/08/10 17:18:00]

Done.

+  virtual base::FilePath GetShaderDiskCacheDirectory();
+
   // 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.