fix: Fix MediaCapabilities polyfill on Safari

Fixes shaka-project#3696
Fixes shaka-project#3530

Change-Id: I2f2e248c9001e10013eb4b03af6b9ef49f28dc6c

Author: joeyparrish

lib/polyfill/media_capabilities.js

@@ -24,8 +24,6 @@ shaka.polyfill.MediaCapabilities = class {
    * @export
    */
   static install() {
-    shaka.log.debug('MediaCapabilities: install');
-
     // Since MediaCapabilities is not fully supported on Chromecast yet, we
     // should always install polyfill for Chromecast.
     // TODO: re-evaluate MediaCapabilities in the future versions of Chromecast.
@@ -40,17 +38,26 @@ shaka.polyfill.MediaCapabilities = class {
     // TODO: re-evaluate MediaCapabilities in the future versions of PS5
     // Browsers.
     if (!shaka.util.Platform.isChromecast() &&
-      !shaka.util.Platform.isApple() &&
-      !shaka.util.Platform.isPS5() &&
-      navigator.mediaCapabilities) {
-      shaka.log.debug(
+        !shaka.util.Platform.isApple() &&
+        !shaka.util.Platform.isPS5() &&
+        navigator.mediaCapabilities) {
+      shaka.log.info(
           'MediaCapabilities: Native mediaCapabilities support found.');
       return;
     }
 
+    shaka.log.info('MediaCapabilities: install');
+
     if (!navigator.mediaCapabilities) {
       navigator.mediaCapabilities = /** @type {!MediaCapabilities} */ ({});
     }
+
+    // Keep the patched MediaCapabilities object from being garbage-collected in
+    // Safari.
+    // See https://github.com/google/shaka-player/issues/3696#issuecomment-1009472718
+    shaka.polyfill.MediaCapabilities.originalMcap =
+        navigator.mediaCapabilities;
+
     navigator.mediaCapabilities.decodingInfo =
         shaka.polyfill.MediaCapabilities.decodingInfo_;
   }
@@ -180,6 +187,16 @@ shaka.polyfill.MediaCapabilities = class {
   }
 };
 
+/**
+ * A copy of the MediaCapabilities instance, to prevent Safari from
+ * garbage-collecting the polyfilled method on it.  We make it public and export
+ * it to ensure that it is not stripped out by the compiler.
+ *
+ * @type {MediaCapabilities}
+ * @export
+ */
+shaka.polyfill.MediaCapabilities.originalMcap = null;
+
 // Install at a lower priority than MediaSource polyfill, so that we have
 // MediaSource available first.
 shaka.polyfill.register(shaka.polyfill.MediaCapabilities.install, -1);