The idea with this patch is that style code will first call
InlineStyleDeclarationWillChange before style declaration has changed, and SetInlineStyleDeclaration once it has changed.
In order to be able to report old attribute value, InlineStyleDeclarationWillChange reads the value and also calls AttributeWillChange (so that DOMMutationObserser can grab the old value). Later SetInlineStyleDeclaration passes the old value to
SetAttrAndNotify so that mutation events and attributeChanged callbacks are handled correctly.
Because of performance, declaration can't be cloned for reading the old value. And that is why the recently-added callback is used to detect when declaration is about to change (bug 1466963 and followup bug 1468665).
To keep the expected existing behavior, even if declaration isn't changed, but just a new declaration was created (since there wasn't any), we need to still run all these
willchange/set calls. That is when the code has 'if (created)' checks.
Since there are several declaration implementation and only nsDOMCSSAttributeDeclaration needs the about-to-change callback, GetPropertyChangeClosure is the one to initialize the callback closure, and the struct which is then passes as data to the closure.
Apparently we lost mutation event testing on style attribute when the pref was added, so test_style_attr_listener.html is modified to test both pref values.
Extract a `CSSPageRule` base class from the existing Gecko `nsCSSPageRule`
class. This will be used in the next commit as the parent of a new
`ServoPageRule` for the Servo variant.
MozReview-Commit-ID: 1F3FWfVKH29
