[Experimental] Supporting OOPIF printing

services/pdf_compositor/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 pdf_compositor.mojom;
+
+interface PdfCompositor {
+  // Add the dependency on a subframe.
+  PrepareSubframe(int32 subframeId, int32 pageNum);
+
+  // Add a subframe content for composition.
+  AddSubFrameContent(int32 subframeId, handle<shared_buffer> sk_handle);
+
+  // Query whether all subframe data are ready.
+  IsReadyToComposite(int32 pageNum) => (bool status);
+
+  // Composite content from one or more subframes, and convert into a PDF file.
+  CompositePdf(handle<shared_buffer> sk_handle)
+      => (handle<shared_buffer> pdf_handle);
+};