Make CSS Calc parsing fail early on unsupported units

This patch fixes an assertion failure in calc parsing when unsupported
primitive value types are used, e.g. calc(1dpi + 2dpi).
Parsing now exits early when these units are encountered.

http://www.w3.org/TR/css3-values/#calc-notation

BUG=366624

Review URL: https://codereview.chromium.org/297633002

git-svn-id: svn://svn.chromium.org/blink/trunk@174479 bbb929c8-8fbe-4397-9dbb-9b2b20218538