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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}