Add yasm to the GN build

content/public/browser/BUILD.gn

 # Copyright 2014 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("//content/browser/browser.gni")
 
 source_set("browser") {
-  sources = rebase_path(content_browser_gypi_values.public_browser_sources,
-                        ".", "//content")
+  if (is_ios) {
+    # iOS doesn't get the normal file list and only takes these whitelisted
+    # files.
+    sources = [
+      "browser_main_parts.cc",
+      "content_browser_client.cc",
+      "favicon_status.cc",
+      "navigation_details.cc",
+      "notification_registrar.cc",
+      "page_navigator.cc",
+      "web_ui_controller.cc",
+    ]
+  } else {
+    sources = rebase_path(content_browser_gypi_values.public_browser_sources,
+                          ".", "//content")
+  }
 
   configs += [ "//content:content_implementation" ]
 
   deps = [
     "//content/browser",
     "//skia",
   ]
 }