Roll src/third_party/pdfium/ d827111fb..a31da74cf (3 commits).

https://pdfium.googlesource.com/pdfium.git/+log/d827111fb10c..a31da74cffa8

$ git log d827111fb..a31da74cf --date=short --no-merges --format='%ad %ae %s'
2016-09-08 tsepez Pass CFX_WideString further down widget callers
2016-09-08 tsepez Remove unused context parameter from JSGetStringFromID
2016-09-08 tsepez Replace CJS_Runtime::Observer with CFX_Runtime<CJS_Runtime>::Observer

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2326683002
Cr-Commit-Position: refs/heads/master@{#417375}

DEPS

@@ -60,7 +60,7 @@ 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': 'd827111fb10c7d8b89bc954ab0934b4009d6643c',
+  'pdfium_revision': 'a31da74cffa8c3ff919051cc49bc006aeb55d345',
   # 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.