ES2016 changed the default timezone of dates to be conditional on
whether a time is included. The semantics were a compromise approach
based on web compatibility feedback from V8, but until now, we have been
shipping ES5.1 default timezone semantics. This patch implements the
new semantics, following ChakraCore and SpiderMonkey (though JSC
implements V8's previous semantics).

BUG=chromium:589858

Review-Url: https://codereview.chromium.org/2648603002
Cr-Commit-Position: refs/heads/master@{#43239}