Move ENABLE_PEPPER_CDMS to a buildflag header.

media/base/BUILD.gn

Index: media/base/BUILD.gn
diff --git a/media/base/BUILD.gn b/media/base/BUILD.gn
index 0c3ad22e1900416492f5a3516dd10457ff1e99f9..1e2954b514f6769f21d7dc539791ff19fbac7cd1 100644
--- a/media/base/BUILD.gn
+++ b/media/base/BUILD.gn
@@ -260,7 +260,9 @@ target(link_target_type, "base") {
 
   allow_circular_includes_from = []
   defines = []
-  public_deps = []
+  public_deps = [
+    "//ppapi/features",
+  ]
   deps = [
     "//base/third_party/dynamic_annotations:dynamic_annotations",
     "//gpu",
@@ -289,7 +291,7 @@ target(link_target_type, "base") {
   }
 
   if (is_android) {
-    public_deps = [
+    public_deps += [
       "//media/base/android",
       "//media/base/android:media_jni_headers",
     ]
@@ -457,6 +459,7 @@ source_set("unittests") {
     "//gpu/command_buffer/common",
     "//media",
     "//media/base/android:anywhere_unittests",
+    "//ppapi/features",
     "//skia",
     "//testing/gmock",
     "//testing/gtest",