Remove support for 'pixel' and 'pos' CSSOM prefixes.

These prefixes are:

1. Non-standard: the CSSOM standard doesn't mention them.

2. Broken: 'pixelWidth' should automagically convert from whatever unit the
   stylesheet specified into CSS pixels[1]. We're currently not doing that at
   all: if an element has a 'width' of 100em, pixelWidth is '100', which is
   probably wildly inaccurate.

3. Not covered by any tests ('fast/dom/CSSStyleDeclaration/css-properties-case-sensitive.html'
   is the only test mentioning these properties, and it only checks
   case-sensitivity: it doesn't validate behavior).

This patch drops support completely.

[1]: http://msdn.microsoft.com/en-us/library/ie/ms531135(v=vs.85).aspx

BUG=265251
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/20968002

git-svn-id: svn://svn.chromium.org/blink/trunk@155313 bbb929c8-8fbe-4397-9dbb-9b2b20218538