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This is a reland of d22a4cef

Fixes:
- Add :wtf_support target for mojo targets, even when only requiring
  headers.
- Slim down :wtf target by only requiring //v8:cppgc_headers when
  using the Oilpan library.

Original change's description:
> wtf: Build traits based on cppgc for when using Oilpan library
>
> Currently WTF uses forward declarations to define basic GC traits that
> are used throughout WTF and Blink globally, i.e., without any specific
> context or other traits being available. This is an implicit
> dependency on blink.
>
> With Oilpan throuhg V8, we make this dependency a header-only
> dependency that is explicit which allows to re-use existing traits
> from the library to define the WTF equivalents.
>
> Change-Id: I3995c0d587424299ed880adcd5ac848a635e4f1e
> Bug: chromium:1056170
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532236
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#826397}

Cq-Include-Trybots: luci.chromium.try:linux-chromeos-rel
Bug: chromium:1056170
Change-Id: Ic9594e7d4824d12cd06273854c9391ee97503fca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532222Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826936}