When interpolating, don't generate a calc expression when one size is zero. 

IE
 calc(0px + 20em) -> 20em
 calc(20px + 0em) -> 20px

This comes up most when interpolating and one size is zero.

The spec has a special case for 0% (which can't be discarded). http://www.w3.org/TR/css3-values/#calc-computed-value
e.g. calc(100% - 100% + 1em) resolves to calc(0% + 1em), not to calc(1em)

Review URL: https://codereview.chromium.org/26194003

git-svn-id: svn://svn.chromium.org/blink/trunk@159748 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make baseline-shift-interpolation easier to eyeball

Review URL: https://codereview.chromium.org/26597003

git-svn-id: svn://svn.chromium.org/blink/trunk@159749 bbb929c8-8fbe-4397-9dbb-9b2b20218538