There are two different limits for the maximum memory size in
WebAssembly:
1) A 4GB limit which is the same on all platforms, and is observable for
JS programs. It is used to limit the allowed declared maximum size of a
wasm memory.
2) A potentially lower limit (2GB on 32-bit systems, 4GB otherwise)
which can be further limited using a command-line flag. This limit is
used whenever actually allocating or growing a wasm memory. This limit
is not directly observable, but we make sure that no wasm memory will
ever be bigger than this limit.

The second limit is the one we should check against when allocating or
growing memory, while the first limit should be used when validating
a module (or the parameters for WebAssembly.Memory). The compiler can
rely on no memory being bigger than the second limit, which again is
never bigger than the first limit.

This CL adds some more documentation to the two limits, and cleans up
all usages.
This also makes {kPlatformMaxPages} and {kMaxMemoryPagesAtRuntime}
obsolete.

R=jkummerow@chromium.org

Bug: chromium:1207263
Change-Id: I43541aafd3f497d1c368bd9400e9bc667bdfd3d9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2910787
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74742}