Move ENABLE_PEPPER_CDMS to a buildflag header.

third_party/widevine/cdm/BUILD.gn

Index: third_party/widevine/cdm/BUILD.gn
diff --git a/third_party/widevine/cdm/BUILD.gn b/third_party/widevine/cdm/BUILD.gn
index a6c45e06cf636a60696f854523f1e0d72e49d51f..8b40b1ab1e7ac238cb695cb177fae873e7cf9eb3 100644
--- a/third_party/widevine/cdm/BUILD.gn
+++ b/third_party/widevine/cdm/BUILD.gn
@@ -7,6 +7,7 @@ import("//build/config/features.gni")
 import("//chrome/process_version_rc_template.gni")
 import("//media/cdm/ppapi/cdm_paths.gni")
 import("//media/cdm/ppapi/ppapi_cdm_adapter.gni")
+import("//ppapi/features/features.gni")
 import("//third_party/widevine/cdm/widevine.gni")
 
 widevine_arch = current_cpu
@@ -52,6 +53,7 @@ if (is_android) {
 }
 
 copy("version_h") {
+  visibility = [ ":*" ]  # Depend on ":headers" instead.
   sources = [
     widevine_cdm_version_h_file,
   ]
@@ -64,6 +66,17 @@ copy("version_h") {
   ]
 }
 
+source_set("headers") {
+  public = [
+    "widevine_cdm_common.h",
+  ]
+
+  public_deps = [
+    ":version_h",  # Forward permission to use version header.
+    "//ppapi/features",
+  ]
+}
+
 if (widevine_cdm_binary_files != []) {
   copy("widevinecdm") {
     sources = widevine_cdm_binary_files