Roll src/third_party/pdfium/ 7855a2c57..c4fcad23b (4 commits).

https://pdfium.googlesource.com/pdfium.git/+log/7855a2c57745..c4fcad23b143

$ git log 7855a2c57..c4fcad23b --date=short --no-merges --format='%ad %ae %s'
2017-01-04 dsinclair [libtiff] Fixup unitialized access issue
2017-01-05 dsinclair Cleaning up memory allocation in CXFA_FM2JSContext - II
2017-01-05 tsepez Banish CFX_Points, CFX_PointsF, and CFX_RectFArray to XFA-side only.
2017-01-05 npm Fix bCJK calculation in Windows MapFont

BUG=677377

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/2615183002
Cr-Commit-Position: refs/heads/master@{#441861}

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': '7855a2c57745c8f81ce410e6af0e4068fa2e8967',
+  'pdfium_revision': 'c4fcad23b1438aa6ad19f518503f861b9e3815e1',
   # 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.