The WasmCompiledModule is kept alive from the Script, which again is
kept alive then the debugger is enabled. This, however, should not keep
the whole context alive, including the global object.
Hence, we only store a weak reference to the native context.

R=ahaas@chromium.org

Bug: chromium:750256
Change-Id: Ia409995c40fb3e90665534fbc94c6eafc081c4e5
Reviewed-on: https://chromium-review.googlesource.com/832126
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50174}