Move file_handlers API from //chrome to //extensions

extensions/browser/api/file_handlers/BUILD.gn

+# Copyright 2017 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.
+
+source_set("file_handlers") {
+  sources = [
+    "app_file_handler_util.cc",
+    "app_file_handler_util.h",
+    "directory_util.cc",
+    "directory_util.h",
+    "mime_util.cc",
+    "mime_util.h",
+  ]
+
+  if (is_chromeos) {
+    sources += [ "non_native_file_system_delegate.h" ]
+  }
+
+  deps = [
+    "//base:base",
+    "//content/public/browser",
+    "//content/public/common",
+    "//extensions/common",
+    "//extensions/common/api",
+    "//net",
+    "//storage/browser",
+  ]
+}