Move ENABLE_PEPPER_CDMS to a buildflag header.

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index c6366d9b01dca14f6d4cc48ca1a3f49357d9bef2..ffc993c44e7d045595386a7c7154f6bb9abb4e73 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -11,6 +11,7 @@ import("//chrome/common/features.gni")
 import("//components/os_crypt/features.gni")
 import("//media/media_options.gni")
 import("//third_party/protobuf/proto_library.gni")
+import("//third_party/widevine/cdm/widevine.gni")
 
 # //build/config/android/rules.gni imports //tools/grit/grit_rule.gni, which
 # produces a conflict for the "grit" template so we have to only include one.
@@ -1445,6 +1446,7 @@ split_static_library("browser") {
     "//third_party/smhasher:cityhash",
     "//third_party/webrtc/modules/desktop_capture",
     "//third_party/widevine/cdm:version_h",
+    "//third_party/widevine/cdm:cdm_features",
     "//third_party/zlib",
     "//third_party/zlib:compression_utils",
     "//third_party/zlib:minizip",