Bug 1479173 - Check mIsRunningOnCompositor flag before iterating LayerAnimationInfo. r=birtles

The comment there was wrong.  We just bail out from there only if
mIsRunningCompositor is false, so it doesn't matter whatever the layer
generation check results.  (i.e., we don't bail out in the case where
mIsRunningCompositor is true).

Also, we iterate over mProperties in the LayerAnimationInfo::sRecords loop
through HasEffectiveAnimationOfProperty, so it doesn't matter that we iterate
mProperties before the loop either.  We will avoid the iteration in the sRecords
loop in a subsequent patch in this series.

Depends on D10692

Differential Revision: https://phabricator.services.mozilla.com/D10693

--HG--
extra : moz-landing-system : lando

dom/animation/KeyframeEffect.cpp

@@ -1242,10 +1242,12 @@ KeyframeEffect::CanThrottle() const
     return true;
   }
 
-  // First we need to check layer generation and transform overflow
-  // prior to the property.mIsRunningOnCompositor check because we should
-  // occasionally unthrottle these animations even if the animations are
-  // already running on compositor.
+  for (const AnimationProperty& property : mProperties) {
+    if (!property.mIsRunningOnCompositor) {
+      return false;
+    }
+  }
+
   for (const LayerAnimationInfo::Record& record :
         LayerAnimationInfo::sRecords) {
     // Skip properties that are overridden by !important rules.
@@ -1278,12 +1280,6 @@ KeyframeEffect::CanThrottle() const
     }
   }
 
-  for (const AnimationProperty& property : mProperties) {
-    if (!property.mIsRunningOnCompositor) {
-      return false;
-    }
-  }
-
   return true;
 }