[MIPS64] Make sure Blink is aware we are 64bit
[MIPS64] Make sure Blink is aware we are 64bit

BUG=400684

Review URL: https://codereview.chromium.org/689703002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@184657 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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