Add error handling and unit test for pdf compositor service

components/printing/service/public/interfaces/pdf_compositor.mojom

 // 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.
 
 module printing.mojom;
 
 const string kServiceName = "pdf_compositor";
 
 // TODO(weili): Add support for printing frames from different processes.
 interface PdfCompositor {
+  // The status of CompositePdf execution.
+  enum Status {
+    SUCCESS,
+    HANDLE_MAP_ERROR,
+    CONTENT_FORMAT_ERROR,
+    COMPOSTING_FAILURE,

[Lei Zhang, 2017/06/02 21:54:09]

type: COMPOSITING_FAILURE
style: kCompositingFailure

[Wei Li, 2017/07/07 21:23:57]

Both styles are ok per style guide
(https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.shtml?cl=head#...)

But mojom files mostly use the former
(https://cs.chromium.org/chromium/src/ipc/ipc.mojom?l=14)

+  };
+
   // Currently directly convert passed in page data to a PDF file.
   // |sk_handle| points to a buffer of a Skia MultiPictureDocument.
-  // |pdf_handle| points to the generated PDF file buffer.
+  // |status| records the function execution status.
+  // |pdf_handle| points to the generated PDF file buffer upon success.
   CompositePdf(handle<shared_buffer> sk_handle)
-      => (handle<shared_buffer> pdf_handle);
+      => (Status status, handle<shared_buffer>? pdf_handle);
 };