Always assume allowed-for-all-content. There are a couple callers which weren't
doing that:
* A unit test -> removed.
* ComputeAnimationDistance: Used for testing (in transitions_per_property), and
for the animation inspector. The animation inspector shouldn't show
non-enabled properties. The transitions_per_property test already relies on
getComputedStyle stuff which only uses eForAllContent.
* GetCSSImageURLs: I added this API for the context menu page and such. It
doesn't rely on non-enabled-everywhere properties, it was only using
eInChrome because it was a ChromeOnly API, but it doesn't really need this.
Differential Revision: https://phabricator.services.mozilla.com/D2514
MozReview-Commit-ID: 4VOi5Su3Bos