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This CL changes the name of a CSS variable. The checkbox spacing was/is
being used for spacing between a control label and something graphical
such as the checkbox or a policy indicator. The intent is to convey
that it's used for more than checkbox spacing.

BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2744763003
Cr-Commit-Position: refs/heads/master@{#456132}