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This makes functioning of FlexLayout a bit more uniform by removing a
conditional of whether the main axis is bounded.  With the addition of
SizeBounds and its various methods, very little syntactic change is needed.

This will also help when fixing bug 1012136.

Bug: 1012136
Change-Id: I0d768bed020c5039ddc326de86aa5b490780c11b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036638
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806898}