Reason for revert:
Broke compilation on component build http://build.chromium.org/p/client.v8/builders/V8%20Win32%20-%20nosnap%20-%20shared/builds/5007/steps/compile/logs/stdio

Original issue's description:
> Add WeakMap 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/37d4c57630636f21e3add8d3d1c7c978ff5fc8e0
> Cr-Commit-Position: refs/heads/master@{#26401}

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

Review URL: https://codereview.chromium.org/898763002

Cr-Commit-Position: refs/heads/master@{#26402}