Add feature policies for controlling an iframe lifecycle.

Two feature policies are added
"allow-execution-while-not-rendered" and
"allow-execution-while-out-of-viewport"

https://github.com/dtapuska/iframe-freeze
https://github.com/WICG/page-lifecycle/pull/32

BUG=907125

Change-Id: I89aeb78ce48b0cacc3c7cc84f4d556a1c431487f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1567893
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654070}
14 files changed