Roll src/third_party/pdfium/ f6d9a61af..06f457216 (14 commits).
Roll src/third_party/pdfium/ f6d9a61af..06f457216 (14 commits).

https://pdfium.googlesource.com/pdfium.git/+log/f6d9a61af218..06f457216f45

$ git log f6d9a61af..06f457216 --date=short --no-merges --format='%ad %ae %s'
2016-05-16 hans Fix binding of null pointer dereference to reference type
2016-05-16 tsepez Remove some c_str() calls from StringCs in xfa docs.
2016-05-16 tsepez Replace FX_WSTRC(L"xyz").c_str() with just L"xyz"
2016-05-16 dsinclair Convert border style defines to an enum class.
2016-05-16 dsinclair Readme update to make need for pdfium_is_standalone clear.
2016-05-13 tsepez Make CFX_ByteString(const CFX_ByteStringC&) explicit.
2016-05-13 tsepez Make CFX_WideString(const CFX_WideString&) explicit.
2016-05-13 weili Fix GCC warnings to be ready for GCC standalone build
2016-05-13 tsepez ReplaceSelections() methods never called.
2016-05-13 tsepez Templatize CFX_{Byte,Wide}StringC
2016-05-13 thestig Make the Skia GN build work.
2016-05-13 thestig Move some classes out of doc_vt.cpp into their own files.
2016-05-13 caryclark allow Skia to be the default graphics engine
2016-05-12 thestig Fix some misc nits.

BUG=none
TBR=thestig

Review-Url: https://codereview.chromium.org/1982763002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393927}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk