Move ENABLE_PEPPER_CDMS to a buildflag header.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 284 matching lines @@
       "//media/capture:capture_unittests",
       "//media/cast:cast_unittests",
       "//media/midi:midi_unittests",
       "//mojo",
       "//mojo/common:mojo_common_unittests",
       "//mojo/edk/system:mojo_system_unittests",
       "//mojo/edk/test:mojo_public_bindings_unittests",
       "//mojo/edk/test:mojo_public_system_unittests",
       "//net:net_perftests",
       "//services/shell/public/cpp",
-      "//third_party/angle/src/tests:angle_end2end_tests",
-      "//third_party/angle/src/tests:angle_unittests",
       "//third_party/WebKit/Source/platform:blink_platform_unittests",
       "//third_party/WebKit/Source/platform/heap:blink_heap_unittests",
       "//third_party/WebKit/Source/web:webkit_unit_tests",
       "//third_party/WebKit/Source/wtf:wtf_unittests",
+      "//third_party/angle/src/tests:angle_end2end_tests",
+      "//third_party/angle/src/tests:angle_unittests",
       "//third_party/catapult/telemetry:bitmaptools($host_toolchain)",
       "//third_party/smhasher:pmurhash",
       "//tools/imagediff($host_toolchain)",
       "//ui/display:display_unittests",
       "//ui/events:events_unittests",
       "//ui/gl:gl_unittests",
       "//ui/touch_selection:ui_touch_selection_unittests",
       "//url/ipc:url_ipc_unittests",
     ]
   } else {
@@ skipping 833 matching lines @@
       "//ui/display:display_unittests",
       "//ui/events:events_unittests",
       "//ui/gfx:gfx_unittests",
       "//ui/gl:gl_unittests",
       "//ui/keyboard:keyboard_unittests",
       "//ui/touch_selection:ui_touch_selection_unittests",
       "//url:url_unittests",
     ]
   }
 }