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This patch adds a uniform mechanism for controlling the viewport insets
for the application. The general approach is such that a feature using
the inset system doesn't have to worry about the specific tab it is
on, only whether there should currently be an inset. In more detail,
this patch does the following:

- The WindowAndroid owns the new ApplicationViewportInsetSupplier.
  - This object hosts basic logic to determine what the window inset
    should be given multiple providers. Currently this is just the
    max(...).
- Tabs register themselves as observers of the AVIS from the
  TabViewAndroidDelegate associated with their web contents.
- The TabViewAndroidDelegate hosts the actual inset functionality
  and reparenting and visibility logic. The tab is ignorant to this.
- The previously public insetViewportBottom has been made private and
  its only user converted to use the new system.

Bug: 1048183
Change-Id: I4e6b63e0e9faf29694dd8957bddd757db5b1f005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064597Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749177}