Roll src/third_party/pdfium/ 574756152..bb2f7e73b (2 commits)

https://pdfium.googlesource.com/pdfium.git/+log/574756152de8..bb2f7e73bef4

$ git log 574756152..bb2f7e73b --date=short --no-merges --format='%ad %ae %s'
2017-09-15 thestig Simplify a couple of CXFA_Node methods.
2017-09-15 rharrison Add ObservedPtrs to KillFocus path

Created with:
  roll-dep src/third_party/pdfium
BUG=760455


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

Change-Id: I5fd7a96a9eaa061fe2efe63434646c9edcb22ecc
Reviewed-on: https://chromium-review.googlesource.com/669307
Reviewed-by: <pdfium-deps-roller@chromium.org>
Commit-Queue: <pdfium-deps-roller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502381}