[Home] Apply bottom margin to native content view widgets

In the cases where a normal web page is not being displayed, the
browser controls are permanently shown (i.e. error page or
interstitial). Because of this, When the browser controls are at
the top of the screen, these views have a top margin applied to
them so the full page can be visible. This code was never updated
for bottom browser controls. This change adds logic to apply a
bottom margin when bottom browser controls exist.

BUG=738616

Change-Id: I155140d8341185b9cdb55f964d2491893732b4dc
Reviewed-on: https://chromium-review.googlesource.com/576268


Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488320}