DEPS.xwalk: Roll v8-crosswalk (edf5696..a91989e)
DEPS.xwalk: Roll v8-crosswalk (edf5696..a91989e)

Required for our GN work in M52.

a91989e Merge pull request #164 from rakuco/backport-gn-fix
d09d8a9 [Backport][gn] Turn on external_startup_data by default except
        on ios

BUG=XWALK-3674
BUG=XWALK-7180

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