Remove value packing and custom style building for text-underline-position.

The user of this property (currently only computeUnderlineOffset) is
oblivious to the fact that RenderStyle::textUnderlinePosition may be a
packed value. (It looks like we'll ASSERT_NOT_REACHED if we actually pack
values during style building).

This was probably done in anticipation of the currently missing left|right
values defined by CSS3 Text Decoration.

I suggest that we remove the custom behavior until support for the
properties are added. When/if support for left|right is added, it should
probably go through StyleBuilderConverter anyway.

Review URL: https://codereview.chromium.org/436563002

git-svn-id: svn://svn.chromium.org/blink/trunk@179384 bbb929c8-8fbe-4397-9dbb-9b2b20218538