Start adding gn build for android_webview

This patch added targets
//android_webview:string_grd
//android_webview:android_webview_java
//android_webview/native:native

BUG=532905

Review URL: https://codereview.chromium.org/1410203002

Cr-Commit-Position: refs/heads/master@{#354869}
diff --git a/components/external_video_surface/BUILD.gn b/components/external_video_surface/BUILD.gn
index 553234a..4698c9d 100644
--- a/components/external_video_surface/BUILD.gn
+++ b/components/external_video_surface/BUILD.gn
@@ -6,9 +6,9 @@
   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",