This ports the configuration for using a sysroot from
chromium's common.gypi.

This is restricted to clang only.

BUG=chromium:474921, chromium:616032
LOG=y

Committed: https://crrev.com/c47d3c63695d2999752f5fc61ff171f3ff100553
Review-Url: https://codereview.chromium.org/2028623002
Cr-Original-Commit-Position: refs/heads/master@{#36729}
Cr-Commit-Position: refs/heads/master@{#36830}