Move chrome.fileSystem implementation to //extensions

extensions/browser/api/file_system/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.
 
 import("//extensions/features/features.gni")
 
 assert(enable_extensions,
        "Cannot depend on extensions because enable_extensions=false.")
 
 source_set("file_system") {
   sources = [
+    "file_system_api.cc",
+    "file_system_api.h",
+    "file_system_delegate.h",
     "saved_file_entry.cc",
     "saved_file_entry.h",
     "saved_files_service_interface.h",
   ]
 
   deps = [
-    "//base:base",
+    "//base",
+    "//content/public/browser",
+    "//extensions/browser/api/file_handlers",
+    "//extensions/common",
+    "//extensions/common/api",
+    "//net",
+    "//storage/browser",
+    "//storage/common",
+    "//ui/base",
+    "//ui/shell_dialogs",
+  ]
+
+  public_deps = [
+    "//extensions/browser:browser_sources",
   ]
 }