Don't draw diagnostic overlays during drawWindow. (bug 1379828, r=mattwoodrow)

gfx/layers/composite/LayerManagerComposite.cpp

@@ -603,6 +603,11 @@ LayerManagerComposite::RenderDebugOverlay(const IntRect& aBounds)
   bool drawFps = gfxPrefs::LayersDrawFPS();
   bool drawFrameColorBars = gfxPrefs::CompositorDrawColorBars();
 
+  // Don't draw diagnostic overlays if we want to snapshot the output.
+  if (mTarget) {
+    return;
+  }
+
   if (drawFps) {
     float alpha = 1;
 #ifdef ANDROID

gfx/layers/mlgpu/LayerManagerMLGPU.cpp

@@ -263,7 +263,8 @@ LayerManagerMLGPU::EndTransaction(const TimeStamp& aTimeStamp, EndTransactionFla
     }
   }
 
-  mDrawDiagnostics = gfxPrefs::LayersDrawFPS();
+  // Don't draw the diagnostic overlay if we want to snapshot the output.
+  mDrawDiagnostics = gfxPrefs::LayersDrawFPS() && !mTarget;
   mUsingInvalidation = gfxPrefs::AdvancedLayersUseInvalidation();
 
   // Compute transforms - and the changed area, if enabled.