fix(drm): Fix application of DRM server certificate

We were previously creating sessions (through filterManifest) before
setting the server certificate (in attach).  This meant the server
certificate was not being used because the license request was created
too early.

Now we will set the server certificate as soon as MediaKeys is
created, ensuring that it is available for all sessions, no matter
when they are created.

Closes shaka-project#2644

Change-Id: I0a04b41c2dee16284375fe1e44b31af9c41cd416

lib/media/drm_engine.js

@@ -412,8 +412,7 @@ shaka.media.DrmEngine = class {
           exception.message));
     });
 
-    const setServerCertificate = this.setServerCertificate();
-    await Promise.all([setMediaKeys, setServerCertificate]);
+    await setMediaKeys;
     this.destroyer_.ensureNotDestroyed();
 
     this.createOrLoad();
@@ -860,6 +859,9 @@ shaka.media.DrmEngine = class {
 
       this.mediaKeys_ = mediaKeys;
       this.initialized_ = true;
+
+      await this.setServerCertificate();
+      this.destroyer_.ensureNotDestroyed();
     } catch (exception) {
       this.destroyer_.ensureNotDestroyed(exception);
 

test/media/drm_engine_unit.js

@@ -653,7 +653,10 @@ describe('DrmEngine', () => {
       config.advanced['drm.abc'] = createAdvancedConfig(cert);
       drmEngine.configure(config);
 
-      await initAndAttach();
+      const variants = manifest.variants;
+      await drmEngine.initForPlayback(variants, manifest.offlineSessionIds);
+
+      // Should be set merely after init, without waiting for attach.
       expect(mockMediaKeys.setServerCertificate).toHaveBeenCalledWith(cert);
     });