Handle line-height values that are calcs

Add a case for calc values in StyleBuilderFunctions::applyValueCSSPropertyLineHeight.

Add test cases to /LayoutTests/css3/calc/line-height.html to cover zoom and calc with ems.

Simplify valueForLineHeight in CSSComputedStyleDeclaration.cpp. This changes behavior so that line-heights are not rounded to integers.

Remove test for line-height rounding. Only percents were rounded previously; with this change nothing is rounded. Rounding isn't mentioned in the specs & Firefox does not round. Original discussion is here: https://bugs.webkit.org/show_bug.cgi?id=24434

BUG=287882

Review URL: https://codereview.chromium.org/50683004

git-svn-id: svn://svn.chromium.org/blink/trunk@161023 bbb929c8-8fbe-4397-9dbb-9b2b20218538