After a certain number of iterations in the fixpoint
iteration switch to a linear algorithm. This
algorithm uses a key-to-values map for weak collections
contents to avoid checking all EphemeronHashTables
again.

Bug: chromium:844008
Change-Id: I044fede5911e0a780d088d1ba2bb5343317d9b7a
Reviewed-on: https://chromium-review.googlesource.com/1105835
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54232}