Remove dead flush histograms code.

content/public/test/mock_render_thread.cc

Index: content/public/test/mock_render_thread.cc
diff --git a/content/public/test/mock_render_thread.cc b/content/public/test/mock_render_thread.cc
index ea5ed2fad362310bc68883eef55093e88690ac17..35fd8dfcd7f54991b4182d1928a855e6518f368a 100644
--- a/content/public/test/mock_render_thread.cc
+++ b/content/public/test/mock_render_thread.cc
@@ -217,9 +217,6 @@ int64_t MockRenderThread::GetIdleNotificationDelayInMs() const {
 void MockRenderThread::SetIdleNotificationDelayInMs(
     int64_t idle_notification_delay_in_ms) {}
 
-void MockRenderThread::UpdateHistograms(int sequence_number) {
-}
-
 int MockRenderThread::PostTaskToAllWebWorkers(const base::Closure& closure) {
   return 0;
 }