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Nested layouts now allocate available space properly to their
descendents. Also, we consistently use flex layout in the PWA titlebar.

Removed a couple of invalid assumptions that were encoded as DCHECKs.

There are still some inkdrop issues but they are purely cosmetic, see
crbug.com/1006162 for more detail.

Bug: 1048061
Change-Id: I403b71b5e2bbcb2d461e1ed6c543c76f91d4c652
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036264
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738874}