Convert enable_plugins to a buildflag header.

chrome/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.
 
 import("//build/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//build/config/locales.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ skipping 304 matching lines @@
       "//chrome/app:command_ids",
       "//chrome/app/theme:chrome_unscaled_resources",
       "//chrome/common:features",
       "//chrome/install_static:install_static_util",
       "//chrome_elf",
       "//components/crash/content/app",
       "//components/policy:generated",
       "//content/app/resources",
       "//crypto",
       "//net:net_resources",
+      "//ppapi/features",
       "//third_party/cld",
       "//third_party/wtl",
       "//ui/views",
     ]
 
     ldflags = [
       "/DELAYLOAD:comdlg32.dll",
       "/DELAYLOAD:crypt32.dll",
       "/DELAYLOAD:cryptui.dll",
       "/DELAYLOAD:dhcpcsvc.dll",
@@ skipping 1276 matching lines @@
       "//chrome/app/theme/$branding_path_component/product_logo_48.png",
       "//chrome/tools/build/linux/chrome-wrapper",
       "//third_party/xdg-utils/scripts/xdg-mime",
       "//third_party/xdg-utils/scripts/xdg-settings",
     ]
     outputs = [
       "$root_out_dir/{{source_file_part}}",
     ]
   }
 }