Enable Purge+throttle by default except Android and MacOS.

purge+throttle is lgtm-ed:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/DK189tnM8l4

The finch result is:
https://docs.google.com/document/d/1L5qRmCwjidiZWKmQRiuqMjZivBWV7XSW9RTypg9XxlI/edit?usp=sharing

BUG=635419

Review-Url: https://codereview.chromium.org/2692313003
Cr-Commit-Position: refs/heads/master@{#460726}
1 file changed