Remove some unnecessary parser mode arguments to validUnit

Currently validUnit takes an optional parser mode, which is used to
determine whether we can accept unitless values as dimensions. It only
matters where we allow lengths, angles, or times, and this patch removes
the usages where it doesn't matter.

BUG=404023

Review URL: https://codereview.chromium.org/509993004

git-svn-id: svn://svn.chromium.org/blink/trunk@181022 bbb929c8-8fbe-4397-9dbb-9b2b20218538