Introduce new flag for starting young generation collection early
based on the current new space size.

Bug: v8:6972
Change-Id: I73dd28b8ac7df873b5c3e6ca4b3e55bdec5295a1
Reviewed-on: https://chromium-review.googlesource.com/811304
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49927}