Fix a CHECK failure when getHTML is disabled

The CHECK was checking the wrong feature for serializable. It
should have checked `DeclarativeShadowDOMSerializable`. This CL
also adds a virtual suite that runs declarative SD tests with
the ElementGetHTML flag disabled, to catch other such cases.

Bug: 41490936
Change-Id: I1271a0e077030f36c5c455caf7b5a30c0615650e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406392


Commit-Queue: Mason Freed <masonf@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280619}