Let gcTree() traverse over shadow boundaries.
Let gcTree() traverse over shadow boundaries.

Before this change, gcTree() doesn't cross shadow boundaries and let
wrappers on shadow DOM tree reclaimed even though these tree can be reachable
through shadow DOM accessors. As a consequence, some ShadowRoot wrappers are
re-created. Such re-creation shouldn never happen.

This change tells gcTree() about shadow boundaries so that it prevents
unexpected reclaim of wrappers.

BUG=240082
TEST=manual browser on https://uitoolkit.googleplex.com/Sandbox/index.html

Review URL: https://chromiumcodereview.appspot.com/14995012

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@150197 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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