Roll PDFium from d257440b81e5 to 55ae76d9fed9 (8 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/d257440b81e5..55ae76d9fed9

2020-06-23 thestig@chromium.org Change enum CPDF_IconFit::ScaleMethod to follow the style guide.
2020-06-22 thestig@chromium.org Change some XFA enums to follow the style guide.
2020-06-22 thestig@chromium.org Add experimental FORM_SelectAllText() API.
2020-06-22 thestig@chromium.org Use std::vector in CFX_SkiaDeviceDriver::PaintStroke().
2020-06-22 thestig@chromium.org Fix some nits in dash drawing code.
2020-06-22 thestig@chromium.org Minor improvements for FXGC code.
2020-06-22 thestig@chromium.org Remove nullptr checks in FreeType deleters.
2020-06-22 thestig@chromium.org Fix typo preventing FXGC_Release() from running.

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/pdfium-autoroll
Please CC pdfium-deps-rolls@chromium.org on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

Bug: None
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I2d233361d7502ac90bbcb618b510d8a03d5b9fe8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2259716Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#781200}

DEPS

@@ -202,7 +202,7 @@ 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': 'd257440b81e54d48ebbb6afd046a9776246c15b6',
+  'pdfium_revision': '55ae76d9fed970d9c944561f8c9d77d7ed5f3e9c',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.