This patch renames the async attribute to be the decoding attribute
due to discussion on https://github.com/whatwg/html/issues/1920

This also adds wpt test for the decoding attribute

[email protected], [email protected]

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I0524b7e3849c9a922f358c0592d34c30a7fd8914
Reviewed-on: https://chromium-review.googlesource.com/770106
Commit-Queue: vmpstr <[email protected]>
Reviewed-by: Chris Harrelson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#519453}