Remove special cases for viewport units, and add tests for viewport units in calc().

The check for viewport units in AnimatableLength::interpolateTo should
no longer be necessary now that viewport values are resolved at style
recalc time.

BUG=137617,168840
TEST=animations/viewport-unit.html
     css3/calc/viewport-unit.html

Review URL: https://codereview.chromium.org/228063006

git-svn-id: svn://svn.chromium.org/blink/trunk@172971 bbb929c8-8fbe-4397-9dbb-9b2b20218538