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
ebca4eacd8