Start adding gn build for android_webview

components/external_video_surface/BUILD.gn

 # Copyright 2015 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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-static_library("external_video_surface") {
+source_set("external_video_surface") {
   if (is_android) {
-    sources += [
+    sources = [
       "browser/android/external_video_surface_container_impl.cc",
       "browser/android/external_video_surface_container_impl.h",
       "component_jni_registrar.cc",
       "component_jni_registrar.h",
     ]
 
     deps = [
       ":jni_headers",
       "//base",
       "//content/public/browser",
     ]
   }
 }
 
 if (is_android) {
   android_library("java") {
     java_files = [ "android/java/src/org/chromium/components/external_video_surface/ExternalVideoSurfaceContainer.java" ]
     deps = [
       "//base:base_java",
       "//content/public/android:content_java",
     ]
   }
   generate_jni("jni_headers") {
     sources = [
       "android/java/src/org/chromium/components/external_video_surface/ExternalVideoSurfaceContainer.java",
     ]
     jni_package = "external_video_surface"
   }
 }