The old algorithm produces unnecessary decimal digits. The new one
converts the significand of the input double into an uint64_t to be
just as precise as necessary.

R=tebbi@chromium.org
BUG=chromium:658712,chromium:666376

Review-Url: https://codereview.chromium.org/2520363002
Cr-Commit-Position: refs/heads/master@{#41255}