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The proper solution is to either way for ios/clean or refactor BVC to use
topLayoutGuide.  Unfortunately, this change will require more than just
replacing usinag of StatusBarHeight, it will also require refactoring various
portions of BVC so layout happens within -viewSeAreaInsetsDidChange.

Instead, take advantage of the fact that, for now, the status bar frame can be
used to fake the topLayoutGuide, and can be used at anytime.

Bug: 
Change-Id: I7b05ce78a0104884fb77d21be512c641b1da0e95
Reviewed-on: https://chromium-review.googlesource.com/669484Reviewed-by: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Jean-François Geyelin <jif@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502756}