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Extension functions should never both set results and an error - it
should always be one or the other. Enforce this more strictly in
the cookies.set function.

Bug: None
Change-Id: I6d75973f78f46d3895abad633d8069a09c14c70e
Reviewed-on: https://chromium-review.googlesource.com/1048395Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556784}