Fix typo in simple-polygon.js LayoutTest helper

The lineHeight parameter to generatePolygonContentString() is mistyped as
'lineheight'.

It looks like this bug has always been present. It doesn't currently create a
problem because in all cases where this function is called, a global variable
named 'lineHeight' exists with the correct value.

See https://src.chromium.org/viewvc/blink?view=revision&revision=130687

Review URL: https://codereview.chromium.org/53583005

git-svn-id: svn://svn.chromium.org/blink/trunk@161112 bbb929c8-8fbe-4397-9dbb-9b2b20218538