fix: Avoid duplicate calls to decodingInfo() (#43)

* refactor: Call 'navigator.requestMediaKeySystemAccess'

* Respond to feedback

* Fix linter errors

* Rework comments to clarify flow

index.js

@@ -301,18 +301,31 @@ class McEncryptionSchemePolyfill {
       return capabilities;
     }
 
-    // If we land here, the browser does _not_ support the mediaKeySystemAccess
-    // field or the encryptionScheme field.  So we install another patch to
-    // check the mediaKeySystemAccess or encryptionScheme field in future calls.
+    // If we land here, either the browser does not support the
+    // encryptionScheme field, or the browser does not support EME-related
+    // fields in MCap _at all_.
+
+    // First, install a patch to check the mediaKeySystemAccess or
+    // encryptionScheme field in future calls.
     console.debug('McEncryptionSchemePolyfill: ' +
         'No native encryptionScheme support found. '+
         'Patching encryptionScheme support.');
     // eslint-disable-next-line require-atomic-updates
     navigator.mediaCapabilities.decodingInfo =
         McEncryptionSchemePolyfill.polyfillDecodingInfo_;
 
-    // The results we have may not be valid.  Run the query again through our
-    // polyfill.
+    // Second, if _none_ of the EME-related fields of MCap are supported, fill
+    // them in now before returning the results.
+    if (!mediaKeySystemAccess) {
+      capabilities.keySystemAccess =
+          await McEncryptionSchemePolyfill.getMediaKeySystemAccess_(
+              requestedConfiguration);
+      return capabilities;
+    }
+
+    // If we land here, it's only the encryption scheme field that is missing.
+    // The results we have may not be valid, since they didn't account for
+    // encryption scheme.  Run the query again through our polyfill.
     return McEncryptionSchemePolyfill.polyfillDecodingInfo_.call(
         this, requestedConfiguration);
   }
@@ -378,16 +391,36 @@ class McEncryptionSchemePolyfill {
           new EmeEncryptionSchemePolyfillMediaKeySystemAccess(
               capabilities.keySystemAccess, supportedScheme);
     } else if (requestedConfiguration.keySystemConfiguration) {
-      const mediaKeySystemConfig =
+      // If the result is supported and the content is encrypted, we should have
+      // a MediaKeySystemAccess instance as part of the result.  If we land
+      // here, the browser doesn't support the EME-related fields of MCap.
+      capabilities.keySystemAccess =
+          await McEncryptionSchemePolyfill.getMediaKeySystemAccess_(
+              requestedConfiguration);
+    }
+
+    return capabilities;
+  }
+
+  /**
+   * Call navigator.requestMediaKeySystemAccess to get the MediaKeySystemAccess
+   * information.
+   *
+   * @param {!MediaDecodingConfiguration} requestedConfiguration The requested
+   *   decoding configuration.
+   * @return {!Promise.<!MediaKeySystemAccess>} A Promise to a
+   *   MediaKeySystemAccess instance.
+   * @private
+   */
+  static async getMediaKeySystemAccess_(requestedConfiguration) {
+    const mediaKeySystemConfig =
           McEncryptionSchemePolyfill.convertToMediaKeySystemConfig_(
               requestedConfiguration);
-      capabilities.keySystemAccess =
+    const keySystemAccess =
           await navigator.requestMediaKeySystemAccess(
               requestedConfiguration.keySystemConfiguration.keySystem,
               [mediaKeySystemConfig]);
-    }
-
-    return capabilities;
+    return keySystemAccess;
   }
 
   /**