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Following the lead of crrev.com/c/2140593, this CL moves the logic
that serializes a child frame's AXTreeID from //content/renderer
into Blink itself, and removes WebElement::GetEmbeddingToken() from
Blink's public API.

Bug: 1094150, 1068668
Change-Id: I7ea4def5c65be6135e7e3c956f4a25071831d506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302890
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789609}