Allocation observers used to set allocation limits with the intention
to statistically sample allocations that crossed those points. Those
limits had random alignment, but since object allocations are always
kTaggedSize-aligned, there is no benefit to having the limit be finer
grained. This patch makes sure that the limit is always aligned, which
in turn implies that the available space in a linear allocation area
is always a multiple of kTaggedSize.

Bug: v8:9700
Change-Id: Ib2980b4b8e792cf516cb734b451862c9e2a98029
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1813026
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63895}