[mpc] Make flexbox properties computed_value_comparable

Bug: 1057072
Change-Id: I1ce62770cc27561d577a1508a87d677fad9da043
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210350


Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772051}