The Web Components v0 APIs have been deprecated since Aug 2018 [1],
disabled by default since Oct 2019 [2], and removed from Origin Trial
support since Oct 2020 [3]. Two of the three WebComponents v0 APIs
(Shadow DOM v0 and Custom Elements v0) were completely removed from
the codebase in Dec 2020 [4]. This API, HTML Imports, which is the
last of the "v0" technologies, was disabled by default in Feb 2021 [5].
Now that that change has stabilized, this CL deletes all remaining
code.

This is being done in one large CL for two reasons:
 1. It is hard to untangle all of the dependencies of this code to
    be able to land this in multiple CLs.
 2. Doing this in a single CL will allow the effects (progressions,
    hopefully, but we'll see) to be more easily tracked.

[1] https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/h-JwMiPUnuU/sl79aLoLBQAJ
[2] https://chromium-review.googlesource.com/c/chromium/src/+/1850795
[3] https://chromium-review.googlesource.com/c/chromium/src/+/2458126
[4] https://chromium-review.googlesource.com/c/chromium/src/+/2586019
[5] https://chromium-review.googlesource.com/c/chromium/src/+/2458206

Marking this fixed! :-)

Bug: 961614
Fixed: 937746
Change-Id: I9c786070e4b374683f400d3cc870bea1608cc7f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2798954
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871167}