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Since most "isAttribDirty()" methods were already const, require it
in the docs and via casting in the generated code to ensure that the
check is idempotent. Mostly for consistency, but this prevents us
from triggering unexpected side effects in future optimizations.

Update the one place (navigator.languages) where a non-const method
was being used.

Review-Url: https://codereview.chromium.org/2884493002
Cr-Commit-Position: refs/heads/master@{#472247}