Move ENABLE_PEPPER_CDMS to a buildflag header.

Removes the global build flag and #define and converts to a buildflag header in
ppapi/features.

A new target "//third_party/widevine/cdm:headers" was added since
widevine_cdm_version.h was unreferenced in the build. Given that this file now
requires //ppapi/features we really need "gn check" to work on it. I made the
existing users of ":version_h" reference this new target which encompasses the
public headers of the widevine CDM.

TBR=jrummell@chromium.org (original reviewer)
TBR=jschuh@chromium.org (render_messages.h owners)
TBR=ddorwin@chromium.org (widevine owners)

Reland of http://crrev.com/2411433002 with official build fix.

Review-Url: https://codereview.chromium.org/2409423002
Cr-Original-Commit-Position: refs/heads/master@{#424687}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c5fcdd0ce8d04569d054322ca21f37272d07005a

config/BUILD.gn

@@ -68,10 +68,6 @@ config("feature_flags") {
   if (enable_notifications) {
     defines += [ "ENABLE_NOTIFICATIONS" ]
   }
-  if (enable_pepper_cdms) {
-    # TODO(brettw) should probably be "=1"
-    defines += [ "ENABLE_PEPPER_CDMS" ]
-  }
   if (enable_plugins) {
     defines += [ "ENABLE_PLUGINS=1" ]
   }

config/features.gni

@@ -125,10 +125,6 @@ use_browser_spellchecker = is_android || is_mac
 # Use Minikin hyphenation engine.
 use_minikin_hyphenation = is_android
 
-# Enables the use of CDMs in pepper plugins.
-enable_pepper_cdms =
-    enable_plugins && (is_linux || is_mac || is_win) && !is_chromecast
-
 # The seccomp-bpf sandbox is only supported on five architectures
 # currently.
 # Do not disable seccomp_bpf anywhere without talking to