Fix bad reference to frameRate on Variant

This is only defined on Stream, not Variant.  This bug has was
introduced in PR shaka-project#2332 and the v2.5.8 release and affects v2.5.8 -
v2.5.10.  The corresponding feature (frameRate restrictions) has
actually never worked.

This was caught by a compiler upgrade which is still in progress.

Change-Id: I0409d9be8d3f6eb0794775ca29f28b98208bab4f

lib/util/stream_utils.js

@@ -141,8 +141,8 @@ shaka.util.StreamUtils = class {
 
     // |variant.frameRate| can be undefined, which breaks
     // the math, so make sure they are there first.
-    if (variant && variant.frameRate) {
-      if (!inRange(variant.frameRate,
+    if (variant && variant.video && variant.video.frameRate) {
+      if (!inRange(variant.video.frameRate,
           restrictions.minFrameRate,
           restrictions.maxFrameRate)) {
         return false;