Fix Decimal.floor() + Decimal.ceiling() for many non-integral values.

Extend the check for an integral value to consider all the digits
being truncated, and not just the rightmost/most-significant one, when
rounding the result towards positive and negative infinity for floor()
and ceiling(), respectively.

R=yosin,mkwst
BUG=234323

Review URL: https://codereview.chromium.org/59603006

git-svn-id: svn://svn.chromium.org/blink/trunk@162197 bbb929c8-8fbe-4397-9dbb-9b2b20218538