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Each DOM node has a corresponding JS wrapper node. In heap snapshot
they appear as duplicates.

Example retaining path with duplicates:
- [1] in InternalNode @2041178784
- [1] in HTMLDivElement @2041079168  // DOM node
- [3] in HTMLDivElement @2231        // JS wrapper
- retainer in Window / @2105

The heap snapshot generator now can merge an embedder node with a node
returned by EmbedderGraph::Node::WrapperNode() function.

This patch implements the WrapperNode() function for each DOM node.

Bug: chromium:811925
Change-Id: I9ea6bc7e45f8ab3d54828b9ac61f7230d04d8019
Reviewed-on: https://chromium-review.googlesource.com/928503Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539132}