Unports parts of https://codereview.chromium.org/1828543003
as per suggestion in
https://codereview.chromium.org/1886293002/

Ports https://codereview.chromium.org/1532723003

Also restores -fmsc-version=1800 to support win-clang with
MSVS 2013.

BUG=chromium:603011
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1895453002

Cr-Commit-Position: refs/heads/master@{#35535}