Auto merge of #16496 - BorisChiou:stylo/transition/trigger, r=heycam

stylo: Bug 1341372 - Detect new transitions and let it run.

These are interdependent patches of Bug 1341372. We let animation-only restyle also work for RESTYLE_CSS_TRANSITIONS, and check if we need to update transitions by each transition property. If it is necessary to create/replace/cancel transitions, we create a SequentialTask for CSS_TRANSITIONS.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix Bug 1341372
- [X] These changes do not require tests because there are tests in Gecko already.

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components/style/properties/helpers/animated_properties.mako.rs

@@ -66,6 +66,19 @@ impl TransitionProperty {
         % endfor
     }
 
+    /// Iterates over every property that is not TransitionProperty::All, stopping and returning
+    /// true when the provided callback returns true for the first time.
+    pub fn any<F: FnMut(TransitionProperty) -> bool>(mut cb: F) -> bool {
+        % for prop in data.longhands:
+            % if prop.animatable:
+                if cb(TransitionProperty::${prop.camel_case}) {
+                    return true;
+                }
+            % endif
+        % endfor
+        false
+    }
+
     /// Parse a transition-property value.
     pub fn parse(input: &mut Parser) -> Result<Self, ()> {
         match_ignore_ascii_case! { &try!(input.expect_ident()),