Do not treat calc() values as percentages

In https://codereview.chromium.org/1405953005 I mistakenly included calc() values
in a path that only works for percentages.

I've added an assert() to prevent others from making this mistake.

BUG=580508

Review URL: https://codereview.chromium.org/1650193002

Cr-Commit-Position: refs/heads/master@{#375039}
(cherry picked from commit 77cbb62c78e1e28d497562f763a47e3820a710ce)

Review URL: https://codereview.chromium.org/1696553004 .

Cr-Commit-Position: refs/branch-heads/2623@{#389}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
4 files changed