Revert of Add WeakKeyMap to v8.h (patchset #2 id:20001 of https://codereview.chromium.org/891473005/)

Reason for revert:
Revert this patch due to shared win build compilation failure

http://build.chromium.org/p/client.v8/builders/V8%20Win32%20-%20nosnap%20-%20shared/builds/5030/steps/compile/logs/stdio

Original issue's description:
> Add WeakKeyMap to v8.h
>
> A new map wich references its keys weakly is added to v8.h. Internally it uses the same storage as JSWeakMap but doesn't depend on the JavaScript part of WeakMap implementation in weak-collection.js, hence it can be instantiated without entering any context.
>
> BUG=chromium:437416
> LOG=Y
>
> Committed: https://crrev.com/ee7ed39ac8327124e74dd7ad5f1de0dede988cb7
> Cr-Commit-Position: refs/heads/master@{#26425}

TBR=jochen@chromium.org,mstarzinger@chromium.org,rossberg@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:437416

Review URL: https://codereview.chromium.org/901663002

Cr-Commit-Position: refs/heads/master@{#26430}