If you asked Canadian Prime Minister Justin Trudeau why we're removing
all implicit LayoutUnit constructors, he'd probably have an easy answer
for you: "It's 2016!"

No functional changes beyond tiny differences in conversions, most to
avoid unncecessary conversions.

BUG=581254

Review URL: https://codereview.chromium.org/1656743002

Cr-Commit-Position: refs/heads/master@{#372856}