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
new file mode 100644
index 0000000000000000000000000000000000000000..4bf80c4d08531438ffadbb352b8920993bab7d3b
--- /dev/null
+++ b/components/printing/service/public/interfaces/pdf_compositor.mojom
@@ -0,0 +1,17 @@
+// 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.

[dcheng, 2017/05/10 07:36:48]

This comment doesn't seem entirely accurate at the moment, it seems like this is
just about turning one skp into a pdf. So for now, maybe let's just keep the
comment about that, and update it when we add subframe support per the TODO?

[Wei Li, 2017/05/11 16:53:02]

Updated.

+  // |sk_handle| points to a buffer of a Skia MultiPictureDocument.
+  // |pdf_handle| points to the generated PDF file buffer.
+  CompositePdf(handle<shared_buffer> sk_handle)
+      => (handle<shared_buffer> pdf_handle);

[dcheng, 2017/05/10 07:36:48]

Realistically, how large do we expect these buffers to be? Is it possible we'll
run into shmem limits by passing this over shared memory?

[Wei Li, 2017/05/11 16:53:02]

For web pages, skia document buffer is smaller than generated pdf file as it is
more compact.
For generated pdf file, the current code without this CL passes it using shared
memory from renderer to browser without problem. In general, for rich webpages
such as nytimes.com, theverge.com, the size is around a few megabytes.

There is one caveat when we load a large pdf file directly, skia redrawing might
generate large output. In this case, we should not call CompositePdf() [as we
don't need to].

+};