Prevent calling internal metrics code with invalid values.

base/metrics/histogram.h

Index: base/metrics/histogram.h
diff --git a/base/metrics/histogram.h b/base/metrics/histogram.h
index 0b946136b0fb7d9046346d1914c6a66e3eb0f094..21902c93bcef230573b087e2eb65db89699808f5 100644
--- a/base/metrics/histogram.h
+++ b/base/metrics/histogram.h
@@ -430,8 +430,15 @@ class BASE_EXPORT Histogram {
   };
 
   //----------------------------------------------------------------------------
-  // minimum should start from 1. 0 is invalid as a minimum. 0 is an implicit
-  // default underflow bucket.
+  // For a valid histogram, input should follow these restrictions:
+  // minimum > 0 (if a minimum below 1 is specified, it will implicitly be
+  //              normalized up to 1)
+  // maximum > minimum
+  // buckets > 2 [minimum buckets needed: underflow, overflow and the range]
+  // Additionally,
+  // buckets <= (maximum - minimum + 2) - this is to ensure that we don't have
+  // more buckets than the range of numbers; having more buckets than 1 per
+  // value in the range would be nonsensical.
   static Histogram* FactoryGet(const std::string& name,
                                Sample minimum,
                                Sample maximum,