perf(Cast): memoize/cache canDisplayType results to reduce startup la…

…tency (#6367)

Related to
#5776 (comment)

lib/polyfill/media_capabilities.js

@@ -300,7 +300,17 @@ shaka.polyfill.MediaCapabilities = class {
       // this query is specifically for HDR.
       displayType += '; eotf=smpte2084';
     }
-    return cast.__platform__.canDisplayType(displayType);
+    let result = false;
+    if (displayType in shaka.polyfill.MediaCapabilities
+        .memoizedCanDisplayTypeRequests_) {
+      result = shaka.polyfill.MediaCapabilities
+          .memoizedCanDisplayTypeRequests_[displayType];
+    } else {
+      result = cast.__platform__.canDisplayType(displayType);
+      shaka.polyfill.MediaCapabilities
+          .memoizedCanDisplayTypeRequests_[displayType] = result;
+    }
+    return result;
   }
 
   /**
@@ -337,6 +347,15 @@ shaka.polyfill.MediaCapabilities.originalMcap = null;
  */
 shaka.polyfill.MediaCapabilities.memoizedMediaKeySystemAccessRequests_ = {};
 
+/**
+ * A cache that stores the canDisplayType result of calling
+ * `cast.__platform__.canDisplayType`.
+ *
+ * @type {(Object<(!string), (!boolean)>)}
+ * @export
+ */
+shaka.polyfill.MediaCapabilities.memoizedCanDisplayTypeRequests_ = {};
+
 // Install at a lower priority than MediaSource polyfill, so that we have
 // MediaSource available first.
 shaka.polyfill.register(shaka.polyfill.MediaCapabilities.install, -1);