Modify build scripts for ios .framework to compile metal shaders.

webrtc/sdk/objc/metal.gni

+# Copyright 2017 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+# Template to compile metal shader files
+#
+# Arguments
+#
+#     source:
+#         string, shader to compile and package in metallib
+template("compile_metal_lib") {
+  action(target_name) {
+    assert(defined(invoker.source),
+           "No sources in $target_name listing the metal shader files.")
+
+    script = "//webrtc/sdk/objc/compile_metal_lib.py"
+    source = invoker.source
+    outputs = [
+      "$target_gen_dir/rtc_shaders.metallib",
+    ]
+    args = [
+      "--input",
+      rebase_path(source),
+      "--output",
+      rebase_path("$target_gen_dir"),
+    ]
+  }
+}