Add a histogram for renderer memory usage.

base/metrics/histogram.h

Index: base/metrics/histogram.h
diff --git a/base/metrics/histogram.h b/base/metrics/histogram.h
index ed8de6f2612e229825ed883042f60b1915578b0e..2c832fd9b17f77e5fad6d86450cb1ecda0acca56 100644
--- a/base/metrics/histogram.h
+++ b/base/metrics/histogram.h
@@ -217,6 +217,9 @@ class Lock;
         base::CustomHistogram::FactoryGet(name, custom_ranges, \
                                           base::HistogramBase::kNoFlags))
 
+#define HISTOGRAM_MEMORY_KB(name, sample) HISTOGRAM_CUSTOM_COUNTS( \
+    name, sample, 1000, 500000, 50)
+
 //------------------------------------------------------------------------------
 // Define Debug vs non-debug flavors of macros.
 #ifndef NDEBUG