fix: Avoid WebCrypto randomUUID when CMCD disabled

If CMCD is disabled, there is no reason to use randomUUID.  And if
tests or applications are running in plain HTTP URLs (what the web
platform calls an "insecure context"), then WebCrypto APIs like
randomUUID don't exist.

If a site or test is running in plain HTTP, and CMCD is disabled
(which is the default), there shouldn't be any need to install the
randomUUID polyfill (which should be optional).

This changes the CMCD utility to skip generating a session ID if CMCD
is disabled.

Change-Id: Iaf9c3fe94bc03c0f5ce46a082d18513ad05fb341

lib/util/cmcd_manager.js

@@ -31,7 +31,7 @@ shaka.util.CmcdManager = class {
      *
      * @private {string}
      */
-    this.sid_ = config.sessionId || window.crypto.randomUUID();
+    this.sid_ = '';
 
     /**
      * Streaming format
@@ -147,6 +147,10 @@ shaka.util.CmcdManager = class {
    */
   applyTextData(request) {
     try {
+      if (!this.config_.enabled) {
+        return;
+      }
+
       this.apply_(request, {
         ot: shaka.util.CmcdManager.ObjectType.CAPTION,
         su: true,
@@ -217,6 +221,9 @@ shaka.util.CmcdManager = class {
    * @private
    */
   createData_() {
+    if (!this.sid_) {
+      this.sid_ = this.config_.sessionId || window.crypto.randomUUID();
+    }
     return {
       v: shaka.util.CmcdManager.Version,
       sf: this.sf_,