Replace variable-length arrays with FixedArrays

Variable-length arrays are not part of the C++ standard, and are
dangerous to use because they allocate a potentially-unbounded
amount of data on the stack. In addition, they trigger a clang warning
(-Wvla-extension) which is currently suppressed in Chromium builds.

This change replaces VLAs with base::FixedArray, which has similar
performance but does not risk overflowing the stack (arrays which are
too large will be allocated on the heap instead).

Bug: 349656479
Change-Id: I30bf62c454ec52cbf4510c720097b816a488176c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5752554
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1336024}