Chromecast: guard PlayReady with C-define to prevent accidental use.

R=lcwu@chromium.org
BUG=400914

Review URL: https://codereview.chromium.org/605923002

Cr-Commit-Position: refs/heads/master@{#297242}

chromecast/media/base/key_systems_common.cc

@@ -13,7 +13,9 @@
 namespace chromecast {
 namespace media {
 
+#if defined(PLAYREADY_CDM_AVAILABLE)
 const char kChromecastPlayreadyKeySystem[] = "com.chromecast.playready";
+#endif  // defined(PLAYREADY_CDM_AVAILABLE)
 
 CastKeySystem GetKeySystemByName(const std::string& key_system_name) {
 #if defined(WIDEVINE_CDM_AVAILABLE)

chromecast/media/base/key_systems_common.h

@@ -10,7 +10,9 @@
 namespace chromecast {
 namespace media {
 
+#if defined(PLAYREADY_CDM_AVAILABLE)
 extern const char kChromecastPlayreadyKeySystem[];
+#endif  // defined(PLAYREADY_CDM_AVAILABLE)
 
 enum CastKeySystem {
   KEY_SYSTEM_NONE = 0,