Expand DynamicPoolSize from IOS test only to also normal function.

Currently if we encounter a HandlePoolAllocFailure we crash, but we
crash so early that we only receive reports if the user is able to
successfully start up later. Thus we don't know if this is common or if
it is only uncommon for users to eventually recover.

This attempts to expand the dynamic pool size feature from being IOS
test only to also be half sized pools to allow iOS to "gracefully"
fallback to smaller pools (they will likely oom at a higher rate).

We also add a UMA histogram to track how frequently this occurs.

Bug: 489679627
Change-Id: Ifdfe1bf5a78f23ac7492532ce22aa1659f7000dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7735705
Commit-Queue: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Thiabaud Engelbrecht <thiabaud@google.com>
Auto-Submit: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1629024}
NOKEYCHECK=True
GitOrigin-RevId: 96a4096d324801bce227aab12c8efe116b2231a6