The implementation sets the rounding mode flag and then uses the
cvtsd2si and cvtsi2sd instructions (convert between float and int) to do
the rounding. Input values outside int range either don't have to be
rounded anyways, or are rounded by calculating input + 2^52 - 2^52 for
positive inputs, or input -2^52 + 2^52 for negative inputs. The original
rounding mode is restored afterwards.

R=titzer@chromium.org

B=575379

Review URL: https://codereview.chromium.org/1584663007

Cr-Commit-Position: refs/heads/master@{#33367}