This CL fixes allocation size alignment violation when allocating store buffer.
If the actual CommitPageSize happens to be bigger than kMinExpectedOSPageSize
we will have a bit of memory wastage but that's a fair trade-off for having
fast store buffer overflow check in write barriers.

Change-Id: I1d775aa8b203cb198e8332477b0bc2befcd9b006
Reviewed-on: https://chromium-review.googlesource.com/c/1351007
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57871}