fix: Handle non existing navigator.platform string (#6517) (#6518)

This fix will make it possible to use v4.8 in Cobalt browser. The
problem is that navigator.platform doesn't exist in Cobalt browser.

Fixes #6517

---------

Co-authored-by: Álvaro Velad Galván <ladvan91@hotmail.com>

lib/util/platform.js

@@ -416,6 +416,9 @@ shaka.util.Platform = class {
       return navigator.userAgentData.platform.toLowerCase() == 'macos';
     }
     // Fall back to the old API, with less strict matching.
+    if (!navigator.platform) {
+      return false;
+    }
     return navigator.platform.toLowerCase().includes('mac');
   }
 
@@ -430,6 +433,9 @@ shaka.util.Platform = class {
       return navigator.userAgentData.platform.toLowerCase() == 'windows';
     }
     // Fall back to the old API, with less strict matching.
+    if (!navigator.platform) {
+      return false;
+    }
     return navigator.platform.toLowerCase().includes('windows');
   }