The memory metric samples memory usage immediately after forcing GC via
LowMemoryNotification. This makes the metric sensitive to the unmapper
tasks timing.

This patch forces eager unmapping in CollectAllAvailableGarbage.

It also forces eager unmapping of non-regular chunks at the beginning
of Mark-Compact to avoid accumulation of non-regular chunks.

Bug: chromium:833291, chromium:826384
Change-Id: Iddf02cd4ab8613385d033899d29525fe6ee47fdd
Reviewed-on: https://chromium-review.googlesource.com/1017102
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52696}