Bug 1363059 - Add a test for images loaded at startup vs. images shown at startup. r=florian,jwatt

This patch enables startupRecorder.js to collect data on
loaded and shown raster and SVG images on startup via events
from native code. It also adds a test that uses this data
to find images that are unnecessarily loaded.

I've not fixed any of the affected images yet, there's a
fairly comprehensive whitelist that I want to gradually
decrease by opening bugs in the respective components.

MozReview-Commit-ID: 9KqQvKLtZhu

image/VectorImage.cpp

@@ -1025,6 +1025,19 @@ VectorImage::Show(gfxDrawable* aDrawable, const SVGDrawingParameters& aParams)
                              aParams.samplingFilter,
                              aParams.flags, aParams.opacity);
 
+#ifdef DEBUG
+  // Record the image drawing for startup performance testing.
+  if (NS_IsMainThread()) {
+    nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+    if (NS_WARN_IF(obs)) {
+      nsCOMPtr<nsIURI> imageURI = mURI->ToIURI();
+      nsAutoCString spec;
+      imageURI->GetSpec(spec);
+      obs->NotifyObservers(nullptr, "image-drawing", NS_ConvertUTF8toUTF16(spec).get());
+    }
+  }
+#endif
+
   MOZ_ASSERT(mRenderingObserver, "Should have a rendering observer by now");
   mRenderingObserver->ResumeHonoringInvalidations();
 }