Removes the deferred handle reference to the native context that
caused a cyclic dependency, which resulted in a memory leak. Instead of
keeping a reference to the native context, we use a phantom reference
to the WasmCompiledModule in order to get the context.
All foreground tasks are now registered in its own foreground task
manager, in order to make sure that we cancel all scheduled
foreground tasks as soon as the CompilationState is collected.

Bug: chromium:825741
Also-by: ahaas@chromium.org
Change-Id: Id69426a15280a14a1dc3ecd035415e7cfa61780b
Reviewed-on: https://chromium-review.googlesource.com/982622Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Kim-Anh Tran <kimanh@google.com>
Cr-Commit-Position: refs/heads/master@{#52270}