Convert enable_plugins to a buildflag header.

media/base/mac/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 assert(is_mac || is_ios)
 
 source_set("mac") {
   # Note: This source_set is depended on only by //media. In the component
   # build, if other component targets also depend on it, multiple copies of
   # the ObjC classes declared in the files below will cause warnings at
@@ skipping 20 matching lines @@
     libs = [
       "AVFoundation.framework",
       "CoreVideo.framework",  # Required by video_frame_mac.cc.
     ]
   }
   set_sources_assignment_filter(sources_assignment_filter)
   configs += [
     "//media:media_config",
     "//media:media_implementation",
   ]
+
+  deps = [
+    "//ppapi/features",
+  ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "video_frame_mac_unittests.cc",
   ]
   libs = [ "CoreVideo.framework" ]
   configs += [ "//media:media_config" ]
   deps = [
     "//testing/gtest",
   ]
 }