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As a proof of concept for the separation of layout generation and
application, this CL reimplements FlexLayout using GetProposedLayout(),
with significant code savings. The new layout has almost the same
programming interface, and passes all existing unit tests.

(Note that this also removes some of the explicit caching in
FlexLayout but it can always be added back in later if we determine
it's needed.)

For more information on the separation, see this upstream CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1653898

And this design doc (internal only for now):
https://docs.google.com/document/d/1fZOg120fswUdaV7tfZh8cXIlcvAL1Gg70GVrsQVXl3g/edit

Next steps:
 - (optional) Expand FlexLayout with some features that make it
   friendlier to use, especially in interpolating layouts.
 - Implement AnimatingLayout (extending LayoutManager) with basic layout
   animations, analogous to those supported by Cocoa's Core Animation
   library.
 - Move toolbar, bookmarks, and tab handles to an animating layout.

Follow-ups:
 - Combine LayoutManager and LayoutManagerBase
 - Update Box, Grid, and Fill layouts
 - Ensure other one-off layouts work with the new framework

Change-Id: I1bbd739e1b2f5e45d92c8d3a38473319c4b6806f
Bug: 898632
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656249
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676086}