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Show the existing browser sync consent dialog when the first browser
window opens. Long-term we will add a first-run "welcome" WebUI,
which will instantiate the helper. For now this unblocks development
on other browser sync settings UI.

Basically, the parts that observe the browser list are speculative,
everything else we will probably keep.

Screenshot: http://screen/CLjXCGd5SQn

Bug: 1013466
Test: added to unit_tests
Change-Id: I8510527b692daf42e3a87e40f4fc5ca36c7f8993
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930946Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726982}