Bug 968923 - part 3c - miscellaneous telemetry changes for use counters; r=gfritzsche

This patch teaches gen-histogram-enum.py how to correctly deal with the
situation where there are no use counters.  It also adds
mozilla::Telemetry::GetHistogramName for debugging purposes.

toolkit/components/telemetry/Telemetry.cpp

@@ -3751,6 +3751,13 @@ GetHistogramById(ID id)
   return h;
 }
 
+const char*
+GetHistogramName(Telemetry::ID id)
+{
+  const TelemetryHistogram& h = gHistograms[id];
+  return h.id();
+}
+
 void
 RecordSlowSQLStatement(const nsACString &statement,
                        const nsACString &dbName,

toolkit/components/telemetry/Telemetry.h

@@ -76,6 +76,8 @@ void AccumulateTimeDelta(ID id, TimeStamp start, TimeStamp end = TimeStamp::Now(
  */
 base::Histogram* GetHistogramById(ID id);
 
+const char* GetHistogramName(ID id);
+
 /**
  * Return a raw histogram for keyed histograms.
  */

toolkit/components/telemetry/gen-histogram-enum.py

@@ -59,6 +59,8 @@ def main(output, *filenames):
     if seen_use_counters:
         print("  HistogramUseCounterCount = HistogramLastUseCounter - HistogramFirstUseCounter + 1", file=output)
     else:
+        print("  HistogramFirstUseCounter = 0,", file=output)
+        print("  HistogramLastUseCounter = 0,", file=output)
         print("  HistogramUseCounterCount = 0", file=output)
     print("};", file=output)