Add PDF compositor service

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

Index: components/printing/service/public/interfaces/pdf_compositor.mojom
diff --git a/components/printing/service/public/interfaces/pdf_compositor.mojom b/components/printing/service/public/interfaces/pdf_compositor.mojom
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+// 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.
+// Currently directly convert passed in page data to a PDF file.
+interface PdfCompositor {
+  // Composite content from one or more subframes, and convert into a PDF file.
+  // |sk_handle| points to a buffer of a number of serialized page image data.
+  //             The content is obtained from skia::SerializePages().
+  // |pdf_handle| points to the generated PDF file buffer.
+  CompositePdf(handle<shared_buffer> sk_handle)
+      => (handle<shared_buffer> pdf_handle);
+};