Implement the inert attribute: https://github.com/WICG/inert#tldr

https://discourse.wicg.io/t/inert-attribute/1838/

BUG=692360

Review-Url: https://codereview.chromium.org/2088453002
Cr-Original-Commit-Position: refs/heads/master@{#474153}
Committed: https://chromium.googlesource.com/chromium/src/+/cca67d31f4a0b4ec2c20090a8bfc38c464274ab2
Review-Url: https://codereview.chromium.org/2088453002
Cr-Commit-Position: refs/heads/master@{#474598}
39 files changed