Currently the memory reducer is activated only after the first mark-
compact GC, which triggered after the old generation reaches 8 MB.

That threshold is too large for mobile. This patch adds a heuristic
to activate the memory reducer if the old generation expands by more
than 1 MB after the bootstrap.

Change-Id: Ic38bc6e2fe8887677f764246c45e38d237e49a94
Reviewed-on: https://chromium-review.googlesource.com/c/1425898Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58982}