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These helpers are necessary to make PDF print/save work properly.
However they are not sufficient:
1) PDF SaveAs requires a bit more work to pass in the URL,
  a temporary hack is shown here: http://crrev.com/554893002/
2) PDF Print requires Constrained windows to work with mime-handler view,
  which is being fixed in http://crrev.com/552013005

BUG=411765
Test=Once the changes noted above is done, it should be
possible to print/save PDF from mime-handler-view.
mime-handler-view can be triggered by navigating to a PDF with the
following two chrome flags:
--out-of-process-pdf
--enable-mime-handler-view

Review URL: https://codereview.chromium.org/556723002

Cr-Commit-Position: refs/heads/master@{#294074}