Bug 957915: Support calc() with percentages for table cell and column widths

Expected result: The 2 cells with 50% should, in all fairness, occupy the same horizontal space

width: calc(50%) width: 600px

 

width: 50% width: 600px

Chromium 69.0.3497.92, Epiphany 3.22.7 (WebKit 2.18.6) and Epiphany 3.31.1-12-g4ac2ba624 (WebKitGTK+ 2.22.2) achieve expected result.