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This adds UKM metrics recording the source and target of each
postMessage event.

For cross-process postMessages, this is recorded from the browser side
as the source ID of the source page (navigation). For same-process
postMessages, this is recorded from the renderer side as the source ID
of the source frame (which may be a subframe), which can then be
correlated with the source page (navigation) via the DocumentCreated
event.

Change-Id: I1ba8fcb5d493f97d80e2de4c5c3f031025488310
Bug: 1112491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530774Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827820}