Roll PDFium from aa7048b18b2f to 538433c72064 (8 revisions)

https://pdfium.googlesource.com/pdfium.git/+log/aa7048b18b2f..538433c72064

2020-10-13 thestig@chromium.org Explain why kFuncTag/kClassTag should remain as strings.
2020-10-12 thestig@chromium.org Bump up `kMaxFormLevel`.
2020-10-12 thestig@chromium.org Add a pixel test with many layers of form xobjects.
2020-10-12 tsepez@chromium.org Garbage collect last CFWL_ classes.
2020-10-12 tsepez@chromium.org Comment some fields as allowed for stack only types.
2020-10-12 nigi@chromium.org Disable FPDFFormFillEmbedderTest.FormText on Windows for Skia and SkiaPaths.
2020-10-12 tsepez@chromium.org Put CFWL_FontManager into its own .cpp/.h files
2020-10-12 tsepez@chromium.org Nest CFWL_FontData in CFWL_FontManager.

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: chromium:972999
Tbr: pdfium-deps-rolls@chromium.org
Change-Id: I7d8faeaa07277bb97ca5c60dfb7c953d3b627284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467061Reviewed-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@{#816428}

DEPS

@@ -215,7 +215,7 @@ 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': 'aa7048b18b2f4713ac2d2158e48c14601a58025c',
+  'pdfium_revision': '538433c720648f67cf6eb2f3ba108ffd97df9eb3',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling BoringSSL
   # and whatever else without interference from each other.