Treat animation-iteration-count:0 specially. (Bug 765218) r=bzbarsky

layout/style/nsAnimationManager.cpp

@@ -213,10 +213,10 @@ ElementAnimations::EnsureStyleRuleFor(TimeStamp aRefreshTime,
       // the keyframes.
       NS_ABORT_IF_FALSE(currentIterationCount >= 0.0, "must be positive");
       PRUint32 whichIteration = int(currentIterationCount);
-      if (whichIteration == anim.mIterationCount) {
+      if (whichIteration == anim.mIterationCount && whichIteration != 0) {
         // When the animation's iteration count is an integer (as it
         // normally is), we need to end at 100% of its last iteration
-        // rather than 0% of the next one.
+        // rather than 0% of the next one (unless it's zero).
         --whichIteration;
       }
       double positionInIteration =