Add v8_external_startup_data GN rule

The new GN rule will enable removal of code which copies natives_blob.bin and
snapshot_blob.bin in Android GN code (for instance "copy_content_shell_assets").

BUG=547162
LOG=Y

Review URL: https://codereview.chromium.org/1412333005

Cr-Commit-Position: refs/heads/master@{#31835}

BUILD.gn

@@ -7,6 +7,10 @@ import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
+if (is_android) {
+  import("//build/config/android/rules.gni")
+}
+
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
 import("//build_overrides/v8.gni")
@@ -411,6 +415,21 @@ action("d8_js2c") {
          rebase_path(inputs, root_build_dir)
 }
 
+if (is_android) {
+  android_assets("v8_external_startup_data_assets") {
+    if (v8_use_external_startup_data) {
+      deps = [
+        "//v8",
+      ]
+      sources = [
+        "$root_out_dir/natives_blob.bin",
+        "$root_out_dir/snapshot_blob.bin",
+      ]
+      disable_compression = true
+    }
+  }
+}
+
 if (v8_use_external_startup_data) {
   action("natives_blob") {
     visibility = [ ":*" ]  # Only targets in this file can depend on this.