Fix crash in StyleBuilderConverter::convertLineWidth for VH/VX

Add support for the vh and vx units to StyleBuilderConverter::
convertLineWidth thereby preventing the ASSERT_NOT_REACHED in
CSSPrimitiveValue::computeLengthDouble.

Fixes the debug-only crash in ietestcenter/css3/valuesandunits/
units-000.htm and makes us render the vh and vx (div10 and div11)
correctly.

BUG=webkit.org/b/85308
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26567003

git-svn-id: svn://svn.chromium.org/blink/trunk@159184 bbb929c8-8fbe-4397-9dbb-9b2b20218538