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Analogous to similar methods in Point and Size.

These are used to simplify layout computation in a follow-up CL.

Change-Id: If509de376cd1395f1d85aa5abe1121662e94629c
Reviewed-on: https://chromium-review.googlesource.com/c/1351081
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611363}