Add GN template for android_assets(). Use it in content_shell_apk.

content/shell/android/BUILD.gn

Index: content/shell/android/BUILD.gn
diff --git a/content/shell/android/BUILD.gn b/content/shell/android/BUILD.gn
index 4fd3e8552bf168b7cd5d09ce395ac267ba41238a..249ac2243d088c7b910be83515579c22ef059486 100644
--- a/content/shell/android/BUILD.gn
+++ b/content/shell/android/BUILD.gn
@@ -110,6 +110,18 @@ copy_ex("copy_content_shell_assets") {
   }
 }
 
+android_assets("content_shell_assets") {
+  deps = [
+    "//base:icu_assets",
+    "//content/shell:pak",
+    "//gin:v8_snapshot_assets",
+  ]
+  sources = [
+    "$root_build_dir/content_shell.pak",
+  ]
+  enable_compression = false
+}
+
 android_apk("content_shell_apk") {
   testonly = true
   data_deps = [
@@ -118,9 +130,9 @@ android_apk("content_shell_apk") {
   deps = [
     ":content_shell_apk_java",
     ":content_shell_apk_resources",
+    ":content_shell_assets",
     ":content_shell_java",
     ":content_shell_manifest",
-    ":copy_content_shell_assets",
     ":libcontent_shell_content_view",
     "//base:base_java",
     "//content/public/android:content_java",
@@ -133,7 +145,6 @@ android_apk("content_shell_apk") {
   apk_name = "ContentShell"
   android_manifest = content_shell_manifest
   native_libs = [ "libcontent_shell_content_view.so" ]
-  asset_location = content_shell_assets_dir
 
   # TODO(GYP)
   #'extra_native_libs': ['<(SHARED_LIB_DIR)/libosmesa.so'],