Include safe_conversions.h in pdfium_engine.cc

Include safe_conversions.h in pdfium_engine.cc

This is required for IWYU for checked_cast<>() use in this file; this
happened to work because pdfium was leaking some base:: symbols. Fixing
that issue in pdfium unmasked the problem.

Committed: https://crrev.com/c85b8aba626c826fb6860e95b111d7247d987cc0
Cr-Commit-Position: refs/heads/master@{#314461}

[Tom Sepez, 2015-02-04 00:07:34]

tsepez@chromium.org changed reviewers:
+ raymes@chromium.org

[Tom Sepez, 2015-02-04 00:07:35]

Hey Raymes, please review.  Looks like I missed this when landing your change a
few days ago.  Thanks.

[raymes, 2015-02-04 00:12:23]

lgtm

[Tom Sepez, 2015-02-04 00:22:36]

The CQ bit was checked by tsepez@chromium.org

[commit-bot: I haz the power, 2015-02-04 00:23:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/902443003/1

[commit-bot: I haz the power, 2015-02-04 01:24:15]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-04 01:25:24]

Patchset 1 (id:??) landed as
https://crrev.com/c85b8aba626c826fb6860e95b111d7247d987cc0
Cr-Commit-Position: refs/heads/master@{#314461}