Post the browser created callback to the message loop
Post the browser created callback to the message loop

That way, the callback will always be invoked on a browser in the same
state, no matter whether one already existed or not.

BUG=none
R=bauerb@chromium.org

Review URL: https://codereview.chromium.org/1403883003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353716}

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