Bug 1291071 (Part 4) - Clean up Decoder::SpeedHistogram() and related code. r=edwin

image/Decoder.cpp

@@ -268,7 +268,7 @@ Decoder::TakeCompleteFrameCount()
 }
 
 DecoderTelemetry
-Decoder::Telemetry()
+Decoder::Telemetry() const
 {
   MOZ_ASSERT(mIterator);
   return DecoderTelemetry(SpeedHistogram(),
@@ -543,12 +543,5 @@ Decoder::PostError()
   }
 }
 
-Telemetry::ID
-Decoder::SpeedHistogram()
-{
-  // Use HistogramCount as an invalid Histogram ID.
-  return Telemetry::HistogramCount;
-}
-
 } // namespace image
 } // namespace mozilla