This patch adds three telemetry scalars to track how WebP is used. All
of these scalars are updated when we do the MIME type confirmation for
an imgRequest when the first data comes in. We know at this point we
decided to load the given content, so there should be minimal false
positives for data the browser loaded but never displayed.
The first two scalars are merely whether or not WebP was observed. One
is for probes, which are tiny WebP images suggested by the Google WebP
FAQ to probe for different aspects of WebP support (lossy, animated,
etc). We want to count this separately as actual WebP content that the
website wishes us to display. Probes will give a measure of how many
users visit websites that probe for WebP support, and content will give
a measure of how many websites don't care and just give us WebP images
regardless.
The third scalar is intended to give a relative measure of how many WebP
images we are being served relative to all other image types. We expect
the ratio to be small, but it would be good to confirm this from the
data.
