Roll src/third_party/pdfium/ c6c2e36f5..81e1e3fd2 (2 commits).

https://pdfium.googlesource.com/pdfium.git/+log/c6c2e36f59f5..81e1e3fd2d33

$ git log c6c2e36f5..81e1e3fd2 --date=short --no-merges --format='%ad %ae %s'
2016-09-19 tsepez Revert "Make CPDF_IndirectObjectHolder use unique_ptr to objects"
2016-09-19 tsepez Make CPDF_IndirectObjectHolder use unique_ptr to objects

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2356623002
Cr-Commit-Position: refs/heads/master@{#419631}

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': 'c6c2e36f59f54b17d9e2c54a2003166e79ccb5e6',
+  'pdfium_revision': '81e1e3fd2d33478733e47bd007b76fac1a663e74',
   # 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.