Fix BUILD.gn and file existence inconsistency.

components/arc/common/input.mojom

Index: components/arc/common/input.mojom
diff --git a/components/arc/common/input.mojom b/components/arc/common/input.mojom
deleted file mode 100644
index 17ecc4397e36755b4b8dd29d46f07cb394b3d53f..0000000000000000000000000000000000000000
--- a/components/arc/common/input.mojom
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright 2015 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.
-
-module arc.mojom;
-
-interface InputInstance {
-  // Registers a virtual input device on the container side.
-  // |name| is the device name, like "Chrome OS Keyboard".
-  // |device_type| is the device type, like "keyboard".
-  // The virtual device will be reading 'struct input_event's from |fd|.  The
-  // ownership of |fd| will be transferred to the receiver, so the sender must
-  // not close it.
-  RegisterInputDevice@0(string name,
-                        string device_type,
-                        handle fd);
-};