Move debug_devtools 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 83 matching lines @@
   "+third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h",
   "+third_party/WebKit/public/platform/WebReferrerPolicy.h",
   "+third_party/WebKit/public/platform/WebSecurityStyle.h",
   "+third_party/WebKit/public/platform/modules/app_banner/app_banner.mojom.h",
   "+third_party/WebKit/public/platform/modules/budget_service/budget_service.mojom.h",
   "+third_party/WebKit/public/platform/modules/notifications/WebNotificationConstants.h",
   "+third_party/WebKit/public/platform/modules/push_messaging/WebPushPermissionStatus.h",
   "+third_party/WebKit/public/platform/modules/screen_orientation/WebScreenOrientationLockType.h",
   "+third_party/WebKit/public/platform/modules/permissions/permission_status.mojom.h",
   "+third_party/WebKit/public/platform/modules/webshare/webshare.mojom.h",
+  "+third_party/WebKit/public/public_features.h",
   "+third_party/WebKit/public/web/WebCache.h",
   "+third_party/WebKit/public/web/WebContextMenuData.h",
   "+third_party/WebKit/public/web/WebFindOptions.h",
   "+third_party/WebKit/public/web/WebInputEvent.h",
   "+third_party/WebKit/public/web/WebMediaPlayerAction.h",
   "+third_party/WebKit/public/web/WebPluginAction.h",
   "+third_party/WebKit/public/web/WebTextDirection.h",
   "+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',
   ],
 }