Revert "Revert "Move V8ValueCache to new-style weak callbacks""

This reverts commit a26c95a624904a885b706406008e8c27360b9f6e.

(The revert was bad and broke the build)

TBR=jochen@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/101983003

git-svn-id: svn://svn.chromium.org/blink/trunk@163091 bbb929c8-8fbe-4397-9dbb-9b2b20218538