Move ENABLE_PEPPER_CDMS to a buildflag header.

media/BUILD.gn

Index: media/BUILD.gn
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 210c28b623566b08b9afb4cb75443dd471db29e3..57e664d8174c9ab22db5484a4ac8c1ebcf160430 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -12,6 +12,7 @@ import("//media/media_options.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/ffmpeg/ffmpeg_options.gni")
+import("//third_party/widevine/cdm/widevine.gni")
 
 buildflag_header("media_features") {
   header = "media_features.h"
@@ -606,6 +607,7 @@ test("media_unittests") {
     "//testing/gtest",
     "//third_party/libwebm",
     "//third_party/libyuv",
+    "//third_party/widevine/cdm:cdm_features",
     "//third_party/widevine/cdm:version_h",
     "//ui/gfx:test_support",
     "//url",