Roll src/third_party/pdfium/ 48f776f7e..7855a2c57 (3 commits).

https://pdfium.googlesource.com/pdfium.git/+log/48f776f7e801..7855a2c57745

$ git log 48f776f7e..7855a2c57 --date=short --no-merges --format='%ad %ae %s'
2017-01-05 dsinclair Cleaning up memory allocation in CXFA_FM2JSContext - I
2017-01-05 tsepez Properly ref-count CFGAS_GEFont with CFX_RetainPtr.
2017-01-05 tsepez Remove CFX_MapPtrToPtr from xfa/fxfa.

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/2617513005
Cr-Commit-Position: refs/heads/master@{#441813}

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': '48f776f7e801d719683b251dc21ee8c0e3250d90',
+  'pdfium_revision': '7855a2c57745c8f81ce410e6af0e4068fa2e8967',
   # 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.