Parser: Make skipping HTML comments optional (Chromium issue 573887).

[es7] implement exponentiation operator proposal (issue 3915).

Performance and stability improvements on all platforms.

Cr-Commit-Position: refs/heads/5.1.173@{#1}
Cr-Branched-From: 3245d74490933aeda9cf3fc79e216d79df52aa3a-refs/heads/master@{#34905}