Roll src/third_party/pdfium/ 6745f96fa..dc39e377e (1 commit).

https://pdfium.googlesource.com/pdfium.git/+log/6745f96fab41..dc39e377e1a3

$ git log 6745f96fa..dc39e377e --date=short --no-merges --format='%ad %ae %s'
2017-01-04 tsepez Add missing operator<() to CFX_RetainPtr.

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, see:
http://www.chromium.org/developers/tree-sheriffs/sheriff-details-chromium#TOC-Failures-due-to-DEPS-rolls

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2614793002
Cr-Commit-Position: refs/heads/master@{#441470}

DEPS

@@ -64,7 +64,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.
-  'pdfium_revision': '6745f96fab41e2b46f57a3717b034a4064c0de02',
+  'pdfium_revision': 'dc39e377e1a3923e63569020beb29f9662c1a3ee',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling openmax_dl
   # and whatever else without interference from each other.