Move ENABLE_PEPPER_CDMS to a buildflag header.

content/DEPS

 # Do NOT add chrome to the list below. We shouldn't be including files
 # from src/chrome in src/content.
 include_rules = [
   # The subdirectories in content/ will manually allow their own include
   # directories in content/ so we disallow all of them.
   "-content",
   "+content/app/resources/grit/content_resources.h",
   "+content/common",
   "+content/grit",
   "+content/public/common",
@@ skipping 61 matching lines @@
   "+third_party/ocmock",
   "+third_party/re2",
   "+third_party/skia",
   "+third_party/sqlite",
   "+third_party/khronos",
   "+third_party/webrtc",
   "+third_party/webrtc_overrides",
   "+third_party/zlib/google",
   "+third_party/WebKit/public/platform",
   "+third_party/WebKit/public/web",
+  "+third_party/widevine/cdm",
 
   "+ui/accelerated_widget_mac",
   "+ui/accessibility",
   "+ui/android",
   # Aura is analogous to Win32 or a Gtk, so it is allowed.
   "+ui/aura",
   "+ui/base",
   "+ui/compositor",
   "+ui/display",
   "+ui/events",
@@ skipping 30 matching lines @@
   "+jni",
 ]
 
 # content -> content/shell dependency is not allowed, except for browser tests.
 specific_include_rules = {
   ".*_browsertest[a-z_]*\.(cc|h)": [
     "+content/shell/browser",
     "+content/shell/common",
   ],
 }