Add <meta name=timeout content=long> to WPT in SlowTests

This is the 2nd step in the process of banning external/wpt from
SlowTests. After cleaning up the wtp entries we're marking them as
slow by using wpt idioms, i.e.:

1) Add <meta name="timeout" content="long"> to .html files
2) Add // META: timeout=long to .js files

Bug: 765026
Change-Id: Iaf051104f419524916e19324996cc36ee37d5426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497005
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643402}
226 files changed