Convert ios_web_view_shell to pure Objective-C.

ios/web_view/shell/BUILD.gn

 # Copyright 2016 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("//tools/grit/repack.gni")
 import("//build/config/ios/rules.gni")
 
 ios_app_bundle("ios_web_view_shell") {
   configs += [ "//build/config/compiler:enable_arc" ]
   deps = [
     "//base",
     "//ios/web_view",
   ]
   bundle_deps = [
     ":resources",
     ":packed_resources",
   ]
   info_plist = "Info.plist"
   sources = [
     "shell_app_delegate.h",
-    "shell_app_delegate.mm",
+    "shell_app_delegate.m",
     "shell_delegate.h",
-    "shell_delegate.mm",
-    "shell_exe_main.mm",
+    "shell_delegate.m",
+    "shell_exe_main.m",
     "shell_view_controller.h",
-    "shell_view_controller.mm",
+    "shell_view_controller.m",
     "translate_controller.h",
-    "translate_controller.mm",
+    "translate_controller.m",
   ]
   libs = [
     "CFNetwork.framework",
     "CoreFoundation.framework",
     "CoreGraphics.framework",
     "CoreText.framework",
     "Foundation.framework",
     "ImageIO.framework",
     "MobileCoreServices.framework",
     "Security.framework",
@@ skipping 34 matching lines @@
   public_deps = [
     ":repack_resources",
   ]
   sources = [
     "$target_gen_dir/web_view_resources.pak",
   ]
   outputs = [
     "{{bundle_resources_dir}}/{{source_file_part}}",
   ]
 }