The deserialization of the {Scope::asm_module} predicate relies on a
context being present for such modules. This ensures we always allocate
such a context, even in cases where no variables are allocated in it.

R=neis@chromium.org
TEST=cctest/test-parsing/AsmModuleFlag
BUG=v8:5653

Review-Url: https://codereview.chromium.org/2561103004
Cr-Commit-Position: refs/heads/master@{#41611}