fix: Avoid setting global Cast hook

For applications that want to handle Cast on their own, we should
avoid setting the global Cast hook.  Now it will only be set if the UI
is configured with a Cast receiver ID or if the application uses
CastProxy explicitly (with or without the UI).

If an application tried to use our Cast features after installing
their own hook, but before the Cast SDK is ready, we will now issue a
warning to alert the application developer of the issue.

Closes shaka-project#3167

Change-Id: Ic3f57ff257652c057f8322811c8db1d5ad3a8463

lib/cast/cast_sender.js

@@ -171,14 +171,36 @@ shaka.cast.CastSender = class {
   init() {
     const CastSender = shaka.cast.CastSender;
 
-    // Check for the cast API and the receiver id.
-    if (!window.chrome || !chrome.cast || !chrome.cast.isAvailable ||
-        !this.receiverAppId_.length) {
-      // Return if either:
-      // 1) The API is not available on this platform or is not ready yet.
-      // 2) No cast receiver id has been provided.
-      // If the API becomes available before this instance dies, or the reveiver
-      // id changes, init() will be called again.
+    if (!this.receiverAppId_.length) {
+      // Return if no cast receiver id has been provided.
+      // Nothing will be initialized, no global hooks will be installed.
+      // If the receiver ID changes before this instance dies, init will be
+      // called again.
+      return;
+    }
+
+    // Check for the cast API.
+    if (!window.chrome || !chrome.cast || !chrome.cast.isAvailable) {
+      // If the API is not available on this platform or is not ready yet,
+      // install a hook to be notified when it becomes available.
+      // If the API becomes available before this instance dies, init will be
+      // called again.
+
+      // A note about this value: In our testing environment, we load both
+      // uncompiled and compiled code.  This global callback in uncompiled mode
+      // can be overwritten by the same in compiled mode.  The two versions will
+      // each have their own instances_ map.  Therefore the callback must have a
+      // name, as opposed to being anonymous.  This way, the CastSender tests
+      // can invoke the named static method instead of using a global that could
+      // be overwritten.
+      if (!window.__onGCastApiAvailable) {
+        window.__onGCastApiAvailable = shaka.cast.CastSender.onSdkLoaded_;
+      }
+      if (window.__onGCastApiAvailable != shaka.cast.CastSender.onSdkLoaded_) {
+        shaka.log.alwaysWarn('A global Cast SDK hook is already installed! ' +
+          'Shaka Player will be unable to receive a notification when the ' +
+          'Cast SDK is ready.');
+      }
       return;
     }
 
@@ -752,11 +774,3 @@ shaka.cast.CastSender.onSdkLoaded_ = (loaded) => {
     }
   }
 };
-
-// The cast SDK will invoke this global callback.  Since in our testing
-// environment, we load both uncompiled and compiled code, this global callback
-// in uncompiled mode can be overwritten by the same in compiled mode.  The two
-// versions will each have their own instances_ map.
-// Therefore we gave the callback a name that the CastSender tests could invoke
-// instead of using a global that could be overwritten.
-window.__onGCastApiAvailable = shaka.cast.CastSender.onSdkLoaded_;