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Before this CL, ModuleTreeLinker::Instantiate assumed that it is only called for
modulator with a valid context. However, asynchronous module graph node load
completion may be triggered after the context was destroyed.

This CL fixes the issue by making ModuleTreeLinker::Instantiate fail without crashing
if the context is invalid.

BUG=594639,716935

Review-Url: https://codereview.chromium.org/2886593002
Cr-Commit-Position: refs/heads/master@{#472246}