media: Support creation and SetCdm() for mojo based CDM.

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 ec2b633b3a3342e5457dcc68c6e8c610ae8125be..b53fc7d3a5492b971a67072feb539c2e1bdcfab5 100644
--- a/media/base/cdm_context.cc
+++ b/media/base/cdm_context.cc
@@ -6,9 +6,17 @@
 
 namespace media {
 
-CdmContext::CdmContext() {}
+CdmContext::CdmContext() {
+}
+
+CdmContext::~CdmContext() {
+}
 
-CdmContext::~CdmContext() {}
+CdmContextProvider::CdmContextProvider() {
+}
+
+CdmContextProvider::~CdmContextProvider() {
+}
 
 void IgnoreCdmAttached(bool success) {
 }