Cookie Store: disallow '=' in cookies with empty names

Per the explainer[1] and tests: "Cookies with an empty name cannot be
set using values containing = as this would result in ambiguous
serializations in the majority of current browsers."

Also, fix a test glitch now that this restriction is implemented.

[1] https://github.com/WICG/cookie-store/blob/gh-pages/explainer.md

Bug: 729800
Change-Id: I9ed02885c217cbdb4c86d8fd236d49c6c56b6e96
Reviewed-on: https://chromium-review.googlesource.com/994145
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548563}
4 files changed