Factor out a Histogram class from HistogramTimer, and use it to measure external fragmentation

src/v8-counters.cc

Index: src/v8-counters.cc
diff --git a/src/v8-counters.cc b/src/v8-counters.cc
index c6aa9cb7f8a3ed8bce78dd03628f9af25c353c70..b020e22f44e917d3e68c4a3c75e97539bfe9c69d 100644
--- a/src/v8-counters.cc
+++ b/src/v8-counters.cc
@@ -34,11 +34,17 @@ namespace internal {
 
 Counters::Counters() {
 #define HT(name, caption) \
-    HistogramTimer name = { #caption, NULL, false, 0, 0 }; \
+    HistogramTimer name = { {#caption, 0, 10000, 50, NULL, false}, 0, 0 }; \
     name##_ = name;
     HISTOGRAM_TIMER_LIST(HT)
 #undef HT
 
+#define HM(name, caption) \
+    Histogram name = { #caption, 1000, 500000, 50, NULL, false }; \
+    name##_ = name;
+    HISTOGRAM_MEMORY_LIST(HM)
+#undef HM
+
 #define SC(name, caption) \
     StatsCounter name = { "c:" #caption, NULL, false };\
     name##_ = name;