[Chromecast] Enable Widevine persistent license support

Widevine already supports persistent licenses. This patch turns on the
flag in EME which allows JS to use persistent license APIs.

BUG=internal b/72744302
TEST=Test page at
https://storage.googleapis.com/m3u8_lag/persistent_license/index.html

Change-Id: If2a4546dbc156d727edf7555412e1275f4cf134b
Reviewed-on: https://chromium-review.googlesource.com/898182Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534434}

chromecast/renderer/media/key_systems_cast.cc

@@ -146,12 +146,10 @@ void AddCmaKeySystems(
   using Robustness = cdm::WidevineKeySystemProperties::Robustness;
 
   key_systems_properties->emplace_back(new cdm::WidevineKeySystemProperties(
-      codecs,                     // Regular codecs.
-      Robustness::HW_SECURE_ALL,  // Max audio robustness.
-      Robustness::HW_SECURE_ALL,  // Max video robustness.
-      enable_persistent_license_support
-          ? EmeSessionTypeSupport::SUPPORTED
-          : EmeSessionTypeSupport::NOT_SUPPORTED,  // persistent-license.
+      codecs,                                // Regular codecs.
+      Robustness::HW_SECURE_ALL,             // Max audio robustness.
+      Robustness::HW_SECURE_ALL,             // Max video robustness.
+      EmeSessionTypeSupport::SUPPORTED,      // persistent-license.
       EmeSessionTypeSupport::NOT_SUPPORTED,  // persistent-release-message.
       // Note: On Chromecast, all CDMs may have persistent state.
       EmeFeatureSupport::ALWAYS_ENABLED,    // Persistent state.