Make units-000.htm from ietestcenter pass.

In order to do this I had to:

- Change the incorrect unit vm to vmin which was already done on the IE test
  center site.

- Modify the test to use a font-size of 20px instead of 16px for the Ahem
  font to work around a font rendering issue in Blink.

- Better support for viewport relative lengths for border.

  Viewport relative lengths for the border-width properties translated from
  the number value of the viewport relative units to a pixel length without
  taking the viewport size into account. The code now converts to a px length
  through conversion of a Length object using the RenderView.

BUG=webkit.org/b/85308

Review URL: https://codereview.chromium.org/26802003

git-svn-id: svn://svn.chromium.org/blink/trunk@159363 bbb929c8-8fbe-4397-9dbb-9b2b20218538