Add a mimeHandler extension API.

extensions/common/api/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("//build/config/features.gni")
 import("//build/json_schema_api.gni")
+import("//mojo/public/tools/bindings/mojom.gni")
 import("schemas.gni")
 
 assert(enable_extensions)
 
-# GYP version: extensions/common/api/api.gyp:extensions_api
-json_schema_api("api") {
+json_schema_api("generated_api") {
   schemas = true
   bundle = true
 }
 
+# GYP version: extensions/common/api/api.gyp:extensions_api_mojom
+mojom("mojom") {
+  sources = []
+  sources = [
+    "mime_handler.mojom",
+  ]
+}
+
+# GYP version: extensions/common/api/api.gyp:extensions_api
+source_set("api") {
+  deps = [
+    ":mojom",
+    ":generated_api",
+  ]
+}
+
 # GYP version: extensions/browser/api/api_registration.gyp:extensions_api_registration
 json_schema_api("api_registration") {
   impl_dir = "//extensions/browser/api"
   bundle_registration = true
 
   deps = [
     ":api",
     "//device/serial",
     "//extensions/common/api/cast_channel:cast_channel_proto",
     "//skia",
   ]
 }