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This CL updates the FlexLayoutView with View builder support. It
also removes the use of enums for property ids and adds unittests.

Bug: None
Change-Id: If968a111eb8945f7150cd9df99a889f30156175a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2473238
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817689}