DOM: Retire runtime flag DOMConvenienceAPI

The ParentNode methods (prepend, append) and the ChildNode methods
(before, after, replaceWith) shipped in Chrome 52 or 54
https://www.chromestatus.com/feature/5723067795898368
https://bugs.chromium.org/p/chromium/issues/detail?id=632273

The runtime flag is no longer needed.

BUG=255482

Change-Id: I4f0d555e0a8e402966f25a2d188a387593083fd3
Reviewed-on: https://chromium-review.googlesource.com/663109
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Ojan Vafai <ojan@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502148}
3 files changed