In streaming compilation, we were computing a way too low code size
estimate, since all {WasmFunction::code} fields were still zero when we
were calling {EstimateNativeModuleCodeSize}. This lead to many separate
code spaces being created during compilation, creating significant
performance and memory overhead.

This CL fixes this by passing the code section length when creating the
{NativeModule}. From this, we can compute the code size estimate just as
before.

Drive-by: Rename "functions_count" to "num_functions" in
{ProcessCodeSectionHeader} to be consistent with the declaration.

R=ahaas@chromium.org

Bug: v8:9950
Change-Id: I30a54c01ed24d0dfecb8a4b6d123015f1803ddeb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1903439
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64849}