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/ImageFactory.cpp

@@ -92,6 +92,18 @@ ImageFactory::CreateImage(nsIRequest* aRequest,
   // Compute the image's initialization flags.
   uint32_t imageFlags = ComputeImageFlags(aURI, aMimeType, aIsMultiPart);
 
+#ifdef DEBUG
+  // Record the image load for startup performance testing.
+  if (NS_IsMainThread()) {
+    nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+    if (NS_WARN_IF(obs)) {
+      nsAutoCString spec;
+      aURI->GetSpec(spec);
+      obs->NotifyObservers(nullptr, "image-loading", NS_ConvertUTF8toUTF16(spec).get());
+    }
+  }
+#endif
+
   // Select the type of image to create based on MIME type.
   if (aMimeType.EqualsLiteral(IMAGE_SVG_XML)) {
     return CreateVectorImage(aRequest, aProgressTracker, aMimeType,