Simplify ToV8() for ScriptWrappable

Switch the cases that don't have access to a ScriptState to call
ScriptWrappable::ToV8() directly, instead of going through ToV8Traits.

Remove the ToV8HelperScriptWrappable helpers in to_v8_traits.h, which
are nearly identical to ScriptWrappable::ToV8(), and delegate to

ScriptWrappable: :ToV8() in ToV8Traits<ScriptWrappable>::ToV8().
Change-Id: Ib892f1b9d2e209ec5c3c87988d0f8288b89d0270
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5623432


Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313668}