Initialize GPU shader disk cache on brower thread.

content/public/browser/content_browser_client.cc

Index: content/public/browser/content_browser_client.cc
diff --git a/content/public/browser/content_browser_client.cc b/content/public/browser/content_browser_client.cc
index 096e7fd9661d0fe8f997003d348f737adc771ad6..5c4acbaaacb2559b95a36df286ece756af45a7e0 100644
--- a/content/public/browser/content_browser_client.cc
+++ b/content/public/browser/content_browser_client.cc
@@ -291,6 +291,10 @@ std::string ContentBrowserClient::GetDefaultDownloadName() {
   return std::string();
 }
 
+base::FilePath ContentBrowserClient::GetShaderDiskCacheDirectory() {
+  return base::FilePath();
+}
+
 BrowserPpapiHost*
     ContentBrowserClient::GetExternalBrowserPpapiHost(int plugin_process_id) {
   return nullptr;