Revert of Moving webrtc.gni up one level from build/

BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
-import("webrtc/webrtc.gni")
+import("webrtc/build/webrtc.gni")
 
 group("default") {
   testonly = true
   deps = [
     "//webrtc",
     "//webrtc/examples",
     "//webrtc/tools",
   ]
   if (rtc_include_tests) {
     deps += [ "//webrtc:webrtc_tests" ]
   }
 }