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This changes long-standing problems with FlexLayout that were not
problems with BoxLayout. Specifically:
 - Flex weights now dictate deviation from a view's preferred size
   (which may now be zero), so that when a view with a FlexLayout
   shrinks, its flex children shrink proportionately, just as they grow
   proportionately when the host view enlarges.
 - When a child view hits its minimum size or drops out, remaining space
   is correctly allocated across other child views, even at the same
   flex priority.

This solves a number of weird corner cases and rounding issues at the
result of a bit more complexity. However, the code is more heavily
documented and broken into steps so the net effect is that it should be
relatively maintainable.

Bug: 1012134, 1012113

Change-Id: Idc3059d572b3d165866b44c077c38d483d862596
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2100026
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809245}