Change EncryptionScheme to match latest spec

The current version of the EME spec
(https://w3c.github.io/encrypted-media/) has encryptionScheme as a
DOMString with an additional value allowed ("cbcs-1-9"). Updating
the code to match this.

This also moves the feature to stable now that it has been approved.

With this change the W3C test
(https://wpt.live/encrypted-media/clearkey-check-encryption-scheme.https.html)
passes.

Bug: 1035153
Test: Updated media_blink_unittests, browser_tests, and WPT tests pass
Change-Id: I00cb2ce8a4c4f52f95817471226b5e8129660247

encrypted-media/scripts/check-encryption-scheme.js

@@ -38,4 +38,9 @@ function runTest(config, qualifier)
   promise_test(
       () => checkEncryptionScheme('cbcs'),
       testnamePrefix(qualifier, config.keysystem) + ' support for "cbcs" encryption scheme.');
+
+  promise_test(
+      () => checkEncryptionScheme('cbcs-1-9'),
+      testnamePrefix(qualifier, config.keysystem) +
+          ' support for "cbcs-1-9" encryption scheme.');
 }