Turn all RenderLayer booleans into bit fields

This should help have better padding on MSVC as we had one unsigned
bit field in the class. This was spotted by the style checker while
refactoring the class for another change.

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/19762006

git-svn-id: svn://svn.chromium.org/blink/trunk@154580 bbb929c8-8fbe-4397-9dbb-9b2b20218538