Warn when UTCTiming is missing for live

Without a UTCTiming element in the manifest, a client cannot
synchronize its clock.  This can easily lead to playback failures,
so we warn application developers when we detect this.

Issue #290

Change-Id: Idcb395f5ece67bf2c7d0d984ad277f552d030eb8

lib/dash/dash_parser.js

@@ -452,7 +452,10 @@ shaka.dash.DashParser.prototype.parseManifest_ =
 
   // Cannot return until we calculate the clock offset.
   var timingElements = XmlUtils.findChildren(mpd, 'UTCTiming');
-  return this.parseUtcTiming_(timingElements).then(function(offset) {
+  var isLive = duration == Number.POSITIVE_INFINITY ||
+               (segmentAvailabilityDuration != null &&
+                segmentAvailabilityDuration < Number.POSITIVE_INFINITY);
+  return this.parseUtcTiming_(timingElements, isLive).then(function(offset) {
     // Detect calls to stop().
     if (!this.networkingEngine_)
       return;
@@ -1053,10 +1056,11 @@ shaka.dash.DashParser.prototype.requestForTiming_ = function(uri, method) {
  * Parses an array of UTCTiming elements.
  *
  * @param {!Array.<!Element>} elems
+ * @param {boolean} isLive
  * @return {!Promise.<number>}
  * @private
  */
-shaka.dash.DashParser.prototype.parseUtcTiming_ = function(elems) {
+shaka.dash.DashParser.prototype.parseUtcTiming_ = function(elems, isLive) {
   var Functional = shaka.util.Functional;
   return Functional.createFallbackPromiseChain(elems, function(elem) {
     var scheme = elem.getAttribute('schemeIdUri');
@@ -1088,8 +1092,14 @@ shaka.dash.DashParser.prototype.parseUtcTiming_ = function(elems) {
             'Unrecognized scheme in UTCTiming element', scheme);
         return Promise.reject();
     }
-  }.bind(this))
-  .catch(function() { return 0; });
+  }.bind(this)).catch(function() {
+    if (isLive) {
+      shaka.log.warning(
+          'A UTCTiming element should always be given in live manifests! ' +
+          'This content may not play on clients with bad clocks!');
+    }
+    return 0;
+  });
 };