[WeakMemoryCache] Revert Field-Trial: Make Reference from MemoryCache to Resource weak

This is partial revert of
https://codereview.chromium.org/1915113005.

The modifications in MemoryCache are reverted before going to beta because it
might be not so stable, while other field-trial related changes are kept
because I'm planning further experiment in the next milestone.

BUG=603462

Review-Url: https://codereview.chromium.org/2110823003
Cr-Commit-Position: refs/heads/master@{#403148}
2 files changed