Use enum hack to define kInvalidCdmId.

media/base/cdm_context.cc

Index: media/base/cdm_context.cc
diff --git a/media/base/cdm_context.cc b/media/base/cdm_context.cc
index 2b8d3d819e5cf6ee8b06ed72ca71b9fbf16324fe..248fc22028fe655a1369e1f472b97ccc8444b176 100644
--- a/media/base/cdm_context.cc
+++ b/media/base/cdm_context.cc
@@ -6,8 +6,6 @@
 
 namespace media {
 
-const int CdmContext::kInvalidCdmId = 0;
-
 CdmContext::CdmContext() {}
 
 CdmContext::~CdmContext() {}