Add a histogram for renderer memory usage.

content/renderer/render_thread_impl.h

Index: content/renderer/render_thread_impl.h
diff --git a/content/renderer/render_thread_impl.h b/content/renderer/render_thread_impl.h
index b1b24458562e504f7710aa844dce5bb75bc2279a..f76f004822504abec45aa7c9bf4c21369dbad460 100644
--- a/content/renderer/render_thread_impl.h
+++ b/content/renderer/render_thread_impl.h
@@ -31,6 +31,7 @@ class WebMediaStreamCenterClient;
 }
 
 namespace base {
+class Histogram;
 class MessageLoopProxy;
 class Thread;
 
@@ -266,6 +267,9 @@ class CONTENT_EXPORT RenderThreadImpl : public RenderThread,
     // |histogram_name|, or |histogram_name| if it shouldn't be customized.
     std::string ConvertToCustomHistogramName(const char* histogram_name) const;
 
+    // Custom histograms maintained by the HistogramCustomizer itself.
+    base::Histogram* memory_renderer_used();
+
    private:
     friend class RenderThreadImplUnittest;
 
@@ -283,6 +287,9 @@ class CONTENT_EXPORT RenderThreadImpl : public RenderThread,
     // possible.
     std::set<std::string> custom_histograms_;
 
+    // Custom histograms maintained by this object.
+    base::Histogram* memory_renderer_used_;
+
     DISALLOW_COPY_AND_ASSIGN(HistogramCustomizer);
   };