Simplifying error recovery in selectors.

Like with declarations (https://chromiumcodereview.appspot.com/16387006/) selectors use 2 strategies to handle errors
simultaniously: bison's error and returning NULL values. It almost doubles error recovery code and makes it hard to test.

BUG=

Review URL: https://chromiumcodereview.appspot.com/17177002

git-svn-id: svn://svn.chromium.org/blink/trunk@152538 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed