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* Splits safety check callbacks into 4 distinct events (one per child).
* Aggregates all callback constants on top of safety_check_page.js.
* Updates passwords and extensions callback to expect a UI string.

Bug: 1032584
Change-Id: Ia10c4809da2331fbc648c628469797eff5dd0504
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080266
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748008}