histogram_macros: Added some documentation explaining constant rule.

base/metrics/histogram_macros.h

Index: base/metrics/histogram_macros.h
diff --git a/base/metrics/histogram_macros.h b/base/metrics/histogram_macros.h
index fa23bea720ce3f825a32551c14f2062891cd94bf..ce1811a5a7ae3e88af49b9127c6cf3c54d084325 100644
--- a/base/metrics/histogram_macros.h
+++ b/base/metrics/histogram_macros.h
@@ -111,7 +111,13 @@
 
 //------------------------------------------------------------------------------
 // Provide easy general purpose histogram in a macro, just like stats counters.
-// The first four macros use 50 buckets.
+// Most of these macros use 50 buckets, but check the definition for details.
+//
+// All of these macros must be called with |name| as a runtime constant --- it
+// doesn't have to literally be a constant, but it must be the same string on
+// all calls from a particular call site. If this rule is violated,
+// STATIC_HISTOGRAM_POINTER_BLOCK will DCHECK, and if DCHECKS are disabled, the
+// data will be written to the wrong histogram.
 
 #define LOCAL_HISTOGRAM_TIMES(name, sample) LOCAL_HISTOGRAM_CUSTOM_TIMES( \
     name, sample, base::TimeDelta::FromMilliseconds(1), \