Use subject principal as triggering principal in <img> "src" attribute.
Also get rid of the `BeforeMaybeChangeAttr`/`AfterMaybeChangeAttr` dance:
It makes more logical sense for these effects to happen _after_ the
attribute has actually been changed.
In order to tailor certain security checks to the caller that is
attempting to load a particular piece of content, we need to be able to
attach an appropriate triggering principal to the corresponding requests.
Since most HTML content is loaded based on attribute values, this means
capturing the subject principal of the caller who sets those attributes,
which in turn means we need to make it available to `AfterSetAttr` hooks
on all relevant element types.
We're currently fairly vague and inconsistent about the values we provide to
content policy implementations for requestOrigin and requestPrincipal. In some
cases they're the triggering principal, sometimes the loading principal,
sometimes the channel principal.
Our existing content policy implementations which require or expect a
loading principal currently retrieve it from the context node.
Since no current callers require the principal to be the loading
principal, and some already expect it to be the triggering principal
(which there's currently no other way to retrieve), a choice was made
to pass the triggering principal whenever possible, but use the loading
principal to determine the origin URL.
This removes a lot of the plumbing for having the platform embed itself
through IPC which was required for B2G running the browser as both
shell and browser application.
This implementation is different than Mozilla, not indirectly queueing cancel
tasks to be arbitrarily ordered, but rather synchronously close in reverse
top layer order.
This adds a pseudo-class `:-moz-modal-dialog` to be able to track whether the
dialog is in the top layer in CSS.
This implements default styling for modal dialogs following the HTML standard.
Slightly off-extreme color scheme chosen for unstyled <dialog> elements.
This removes some hackery surrounding preventing content clicks, and in
general handles auxclick as it should, firing that event on secondary
buttons (wheel/right on default setup for right-handed mouse).
This restores support for using HTML tag styling of margins on the body.
Reftests post bug still pass for dynamic changes, so does the newly-added
test for body margins and frames that tests the old styling method.
This also removes the reftest list for stylo testing for obvious reasons.
