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Replaces the custom backlink from V8->Blink that was using a v8::External with a
GC-aware backpointer that is encapsulated in the CustomWrappableAdapter
interface.

CustomWrappableAdapter establishes a link from a given JavaScript object to the
Blink object inheriting from CustomWrappableAdapter. The link is known to
garbage collectors and thus the Blink object will be kept alive as long as the
JavaScript object is alive. The adapter can be used to model liveness across V8
and Blink component boundaries. In contrast to ScriptWrappable, there is no IDL
definitions required and the JavaScript object must not escape to user-visible
JavaScript.

This reverts commit c1d0a59d.

Bug: chromium:843903
Change-Id: I8763cdf56ff3cbeff5fa7ae508bc1be8a5deb1c3
Reviewed-on: https://chromium-review.googlesource.com/1154926Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579039}