Move StyleBuilderConverter into it's own file

This moves the StyleBuilderConverter class out of StyleBuilderCustom
and into it's own file. The converter is a helper class that allows
us to separate the conversion of CSSValues from the application to
RenderStyles.

The StyleBuilderCustom code is primarily to support custom defined
property application (so maybe StyleBuilderFunctionsCustom is a better
name..), but it currently also handles the StyleBuilderConverter,
CSS variables, and some of the setup for the StyleBuilder. We would
ideally split these all up (the setup can go to StyleBuilder.cpp.tmpl,
CSS variables can get a separate resolver class) and this patch moves
us slightly closer to there.

BUG=

Review URL: https://codereview.chromium.org/98093002

git-svn-id: svn://svn.chromium.org/blink/trunk@163066 bbb929c8-8fbe-4397-9dbb-9b2b20218538