Conversion of Date is trivial

Date values are converted using toCoreDate() in V8Bindings.h, which
specifically handles native Date objects and numbers, converting each in
a trivial way, and returning NaN for all other values.

Note: WebIDL actually only allows Date objects, so toCoreDate() supports
too much and would thus be even more trivial if it was to be adjusted to
fully match the specification.

BUG=413257

Review URL: https://codereview.chromium.org/604723002

git-svn-id: svn://svn.chromium.org/blink/trunk@182668 bbb929c8-8fbe-4397-9dbb-9b2b20218538