Reland "PCScan: Allocate mojo::Channel on non-scannable partition""

The change was reverted due to compilation failures on iOS builds. The
fix is to wrap partition_alloc includes in ifdefs.

Original change's description:
> PCScan: Allocate mojo::Channel on non-scannable partition
>
> The CL introduces a non-scannable partition, the purpose of which is
> to exclude objects that are known to not contain pointers and thereby
> speed up the scanning.
>
> mojo::Channel::Message is currently the only type, allocation of which
> is moved to the new partition. All mojo messages amount to roughly 4-5%
> of the overall browser memory (tested in cnn:2020).
>
> Bug: 11297512
> Change-Id: I0159f7c01d36174cba54f710a82aa86672862efc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2678949


> Commit-Queue: Anton Bikineev <bikineev@chromium.org>
> Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
> Reviewed-by: Benoit L <lizeb@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#859600}

Bug: 11297512
Change-Id: I4c3debd5f2459a4cb435ce68884da9320a38f99c
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737076


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Owners-Override: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#860166}