Move enable_webrtc to a buildflag header.

chrome/browser/DEPS

 include_rules = [
   "+apps",
   "+ash",
   "+chrome/app",
   "+chrome/chrome_watcher",
   "+chrome/grit",
   "+chrome/install_static",
   "+chrome/installer",
   "+chrome_elf/blacklist",
   "+chrome_elf/chrome_elf_constants.h",
@@ skipping 18 matching lines @@
   "+extensions/test",
   "+gin",
   "+google/cacheinvalidation",  # Sync invalidation API protobuf files.
   "+google_apis",
   "+google_update",
   "+installer_util_strings",  # For generated headers
   "+jni",
   "+media/audio", # For media audio hang monitor.
   "+media/base",  # For media switches
   "+media/cdm/cdm_paths.h",
-  "+media/media_features.h",
   "+media/midi",  # For midi switches
   "+media/mojo",  # For mojo media services.
   "+services/image_decoder/public/cpp",
   "+services/service_manager/public/cpp",
   "+services/ui/public",
   "+ppapi/c",  # For various types.
   "+ppapi/host",
   "+ppapi/proxy",
   "+ppapi/shared_impl",
   "+rlz",
@@ skipping 69 matching lines @@
   "+third_party/WebKit/public/web/WebWindowFeatures.h",
 ]
 
 specific_include_rules = {
   # Exploits don't abide by DEPS; so all IPC message definitions are fair game
   # in this type of test.
   'chrome_security_exploit_browsertest\.cc' : [
     '+content/common',
   ],
 }