Add a mimeHandler extension API.

extensions/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("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 assert(enable_extensions)
 
@@ skipping 41 matching lines @@
     "grit/extensions_renderer_resources.h",
     "extensions_renderer_resources.pak",
   ]
   grit_flags = [
     "-E",
     "mojom_root=" + rebase_path(root_gen_dir),
   ]
 
   deps = [
     "//device/serial:serial_mojo",
+    "//extensions/common/api:mojom",
   ]
 }
 
 source_set("test_support") {
   testonly = true
   sources = [
     "browser/api/dns/mock_host_resolver_creator.cc",
     "browser/api/dns/mock_host_resolver_creator.h",
     "browser/api/storage/settings_test_util.cc",
     "browser/api/storage/settings_test_util.h",
@@ skipping 210 matching lines @@
       "//third_party/mojo/src/mojo/edk/js",
       "//third_party/mojo/src/mojo/edk/system",
       "//third_party/mojo/src/mojo/public/cpp/bindings",
     ]
 
     if (is_win) {
       deps += [ "//base/allocator" ]
     }
   }
 }