Roll src/third_party/pdfium/ a5b37f6b9..b12fbddec (2 commits).

https://pdfium.googlesource.com/pdfium.git/+log/a5b37f6b916f..b12fbddece43

$ git log a5b37f6b9..b12fbddec --date=short --no-merges --format='%ad %ae %s'
2016-09-19 dsinclair Update DEPS to always pull clang.
2016-09-19 tsepez Clean up CPDF_Stream.

BUG=647380

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2350173003
Cr-Commit-Position: refs/heads/master@{#419526}

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': 'a5b37f6b916f1a35d3fbb3f0b41e9e4b4d265370',
+  'pdfium_revision': 'b12fbddece4339c583524401f9b1465d968b3ae6',
   # 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.