Add ijar to third_party and use it for generating .jar.toc files in GN

third_party/ijar/ijar.gni

+# 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.
+
+import("//build/compiled_action.gni")
+
+# Create an interface jar from a normal jar.
+#
+# Variables
+#   input_jar: Path to input .jar.
+#   output_jar: Path to output .ijar.
+#
+template("generate_interface_jar") {
+  compiled_action(target_name) {
+    forward_variables_from(invoker,
+                           [
+                             "deps",
+                             "testonly",
+                           ])
+    tool = "//third_party/ijar"
+    inputs = [
+      invoker.input_jar,
+    ]
+    outputs = [
+      invoker.output_jar,
+    ]
+    args = [
+      rebase_path(invoker.input_jar, root_build_dir),
+      rebase_path(invoker.output_jar, root_build_dir),
+    ]
+  }
+}