Bug 1128989 - part 2 - forward-declare Telemetry::ID in Decoder.h; r=seth

This change means that Decoder.h (and any files that include it) no
longer depend on Telemetry.h.  And since Telemetry.h can change rather
frequently, this change helps minimize the number of files that need to
be rebuilt when Telemetry.h does change.

image/decoders/nsGIFDecoder2.cpp

@@ -48,6 +48,7 @@ mailing address.
 #include "gfxPlatform.h"
 #include "qcms.h"
 #include <algorithm>
+#include "mozilla/Telemetry.h"
 
 namespace mozilla {
 namespace image {

image/decoders/nsJPEGDecoder.cpp

@@ -19,6 +19,7 @@
 
 #include "gfxPlatform.h"
 #include "mozilla/Endian.h"
+#include "mozilla/Telemetry.h"
 
 extern "C" {
 #include "iccjpeg.h"

image/decoders/nsPNGDecoder.cpp

@@ -15,6 +15,7 @@
 #include "nspr.h"
 #include "png.h"
 #include "RasterImage.h"
+#include "mozilla/Telemetry.h"
 
 #include <algorithm>
 

image/src/Decoder.cpp

@@ -15,6 +15,7 @@
 #include "nsProxyRelease.h"
 #include "nsServiceManagerUtils.h"
 #include "nsComponentManagerUtils.h"
+#include "mozilla/Telemetry.h"
 
 using mozilla::gfx::IntSize;
 using mozilla::gfx::SurfaceFormat;
@@ -701,5 +702,12 @@ Decoder::NeedNewFrame(uint32_t framenum, uint32_t x_offset, uint32_t y_offset,
   mNeedsNewFrame = true;
 }
 
+Telemetry::ID
+Decoder::SpeedHistogram()
+{
+  // Use HistogramCount as an invalid Histogram ID.
+  return Telemetry::HistogramCount;
+}
+
 } // namespace image
 } // namespace mozilla

image/src/Decoder.h

@@ -13,10 +13,13 @@
 #include "ImageMetadata.h"
 #include "Orientation.h"
 #include "SourceBuffer.h"
-#include "mozilla/Telemetry.h"
 
 namespace mozilla {
 
+namespace Telemetry {
+  enum ID : uint32_t;
+}
+
 namespace image {
 
 class Decoder : public IResumable
@@ -263,8 +266,7 @@ public:
     return mImageMetadata.GetSize();
   }
 
-  // Use HistogramCount as an invalid Histogram ID
-  virtual Telemetry::ID SpeedHistogram() { return Telemetry::HistogramCount; }
+  virtual Telemetry::ID SpeedHistogram();
 
   ImageMetadata& GetImageMetadata() { return mImageMetadata; }