I have a project that embeds V8 and uses a single `Isolate` from multiple
threads. The program runs just fine, but sometimes the inspector doesn't
stop on the correct line after stepping over a statement that switches
threads behind the scenes, even though the original thread is restored by
the time the next statement is executed.

After some digging, I discovered that the `Debug::ArchiveDebug` and
`Debug::RestoreDebug` methods, which should be responsible for
saving/restoring this `ThreadLocal` information when switching threads,
currently don't do anything.

This commit implements those methods using MemCopy, in the style of other
Archive/Restore methods in the V8 codebase.

Related: https://groups.google.com/forum/#!topic/v8-users/_Qf2rwljRk8

Note: I believe my employer, Meteor Development Group, has previously
signed the CLA using the group email address google-contrib@meteor.com.

R=yangguo@chromium.org,jgruber@chromium.org
CC=info@bnoordhuis.nl

Bug: v8:7230
Change-Id: Id517c873eb81cd53f7216c7efd441b956cf7f943
Reviewed-on: https://chromium-review.googlesource.com/833260
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54902}