Work around a clang/win bug, add a .h file to gyp/gn files.

media/base/cdm_config.h

Index: media/base/cdm_config.h
diff --git a/media/base/cdm_config.h b/media/base/cdm_config.h
index 4b57ebb96c176982293b1bc137576184dd809d98..6f723485f7b94d8682fc2a4028644582c1508003 100644
--- a/media/base/cdm_config.h
+++ b/media/base/cdm_config.h
@@ -12,7 +12,7 @@ namespace media {
 // The runtime configuration for new CDM instances as computed by
 // |requestMediaKeySystemAccess|. This is in some sense the Chromium-side
 // counterpart of Blink's WebMediaKeySystemConfiguration.
-struct MEDIA_EXPORT CdmConfig {
+struct CdmConfig {
   // Allow access to a distinctive identifier.
   bool allow_distinctive_identifier = false;