Add delegate to install web_contents helpers for MimeHandlerView.
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}