PDFium builds V8 with MSVC and is_debug=true (which is not a
configuration covered by the V8 bots). crrev.com/c/2690588 introduced a
DCHECK_EQ which triggers an MSVC compiler error due to comparing a bool
and a uint8_t value.

This change converts the uint8_t value into a bool using "!!", fixing
the MSVC error. This will unblock V8 rolls into PDFium.

Example build log:
https://logs.chromium.org/logs/pdfium/buildbucket/cr-buildbucket.appspot.com/8851041716432959712/+/steps/compile_with_ninja/0/stdout

Change-Id: I5c365696ead7958709df145ccc2e5f796dec80ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2806743
Auto-Submit: K. Moon <kmoon@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73797}