Fix WHITESPACE issues in the CSS grammar.

A single WHITESPACE token consumes consecutive spaces, but does not consume
spaces separated by comments. That means S* and S+ in CSS grammars need to
accept multiple WHITESPACE tokens. Additionally, white spaces are not
mandatory to separate an @-symbol and the rest of the prelude.

Use space non-terminal instead of WHITESPACE for S+ in calc expressions.

Use maybe_space non-terminal instead of WHITESPACE for S* after @-webkit-filter
and @-webkit-region.

BUG=303303

Review URL: https://codereview.chromium.org/25607005

git-svn-id: svn://svn.chromium.org/blink/trunk@158950 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[CSS Regions] Helper functions for selection layout tests

Patch already landed in WebKit. See http://trac.webkit.org/changeset/156807

Create new common JavaScript methods for CSS Regions layout tests related to
selection. Refactored tests in order to use them.

Review URL: https://codereview.chromium.org/25761004

git-svn-id: svn://svn.chromium.org/blink/trunk@158951 bbb929c8-8fbe-4397-9dbb-9b2b20218538