Remove the experimental CSS Grid Layout settings code

Chromium doesn't require this code anymore. While testing, discovered
that the check in CSSParser was wrong after unprefixing flexbox and
would make us never reach the custom checks for grid and lazy-layout.

BUG=241619

Review URL: https://chromiumcodereview.appspot.com/18580003

git-svn-id: svn://svn.chromium.org/blink/trunk@154521 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert "Make more members of RuleFeatureSet private"

This reverts commit 635fffd038c2f133ff6d81b02abb0ce1fdd38008.

Causes assertion failures in debug builds, e.g.
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20%28dbg%29/builds/10031/steps/webkit_tests/logs/stdio

TBR=eseidel@chromium.org
BUG=237268

Review URL: https://codereview.chromium.org/19520012

git-svn-id: svn://svn.chromium.org/blink/trunk@154522 bbb929c8-8fbe-4397-9dbb-9b2b20218538