For the WeakHashTable objects only mark keys as weak while leaving values as strong references.

BUG=chomium:773722

Change-Id: Iabd5ba293d05fe68a2af6503fcdd711ecc182482
Reviewed-on: https://chromium-review.googlesource.com/730771
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48985}