Use pdf compositor service for printing when OOPIF is enabled

When OOPIF is enabled (by site-per-process flag or
top-document-isolation feature), use the pdf compositor service for
converting PaintRecord to PDF on renderers.

In the future, this will make compositing PDF from multiple renderers
possible.

TBR=jzfeng@chromium.org
BUG=455764

Change-Id: I3c28f03f4358e4228239fe1a33384f85e7716e8f
Reviewed-on: https://chromium-review.googlesource.com/699765
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511616}
std::string HeadlessWebContentsImpl::GetMainFrameDevToolsId() const {
  return web_contents()->GetMainFrame()->GetDevToolsFrameToken().ToString();
}

std::string HeadlessWebContentsImpl::GetMainFrameDevToolsId() const {
  return web_contents()->GetMainFrame()->GetDevToolsFrameToken().ToString();
}
