[ES6 modules] Make ModuleMap TraceWrapperBase

This CL makes ModuleMap a TraceWrapperBase so that it can trace into ModuleScript,
which will eventually keep on to its m_instantiationError via TraceWrapperV8Reference.

This CL is required for https://codereview.chromium.org/2782403002/

BUG=594639

Review-Url: https://codereview.chromium.org/2788513003
Cr-Commit-Position: refs/heads/master@{#462047}
3 files changed