Assert that number|string union is number

The new Closure Compiler complains that a number|string union from a
map in MpdUtils is used in a calculation.  In practice, we know that
this specific value is always a number, so we add an assertion to
satisfy the newer compiler's type checks.

Issue #2528

Change-Id: Id12de47d2dd4a12f9cc35879bee8da5ee25cdd70

lib/dash/mpd_utils.js

@@ -69,6 +69,8 @@ shaka.dash.MpdUtils = class {
       }
 
       if (name == 'Time') {
+        goog.asserts.assert(typeof value == 'number',
+            'Time value should be a number!');
         goog.asserts.assert(Math.abs(value - Math.round(value)) < 0.2,
             'Calculated $Time$ values must be close to integers');
         value = Math.round(value);