[Chromecast] Fix key system build flag logic.

Should use Cast EME with PlayReady or Widevine+CMA.

BUG=internal 62721854
TEST=

Change-Id: Ide4de9e3dea6a1286415569bff68147caeb1cdc6
Reviewed-on: https://chromium-review.googlesource.com/570981Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487130}

chromecast/renderer/media/key_systems_cast.cc

@@ -30,8 +30,8 @@ namespace chromecast {
 namespace media {
 namespace {
 
-#if BUILDFLAG(IS_CAST_USING_CMA_BACKEND) && \
-    (defined(WIDEVINE_CDM_AVAILABLE) || defined(PLAYREADY_CDM_AVAILABLE))
+#if defined(PLAYREADY_CDM_AVAILABLE) || \
+    (BUILDFLAG(IS_CAST_USING_CMA_BACKEND) && defined(WIDEVINE_CDM_AVAILABLE))
 SupportedCodecs GetCastEmeSupportedCodecs() {
   SupportedCodecs codecs =
       ::media::EME_CODEC_MP4_AAC | ::media::EME_CODEC_MP4_AVC1 |
@@ -55,7 +55,7 @@ SupportedCodecs GetCastEmeSupportedCodecs() {
 
   return codecs;
 }
-#endif  // BUILDFLAG(IS_CAST_USING_CMA_BACKEND) && ...
+#endif  // defined(PLAYREADY_CDM_AVAILABLE) || ...
 
 #if defined(PLAYREADY_CDM_AVAILABLE)
 class PlayReadyKeySystemProperties : public ::media::KeySystemProperties {