Don't null check the style resolver constantly inside recalcChildStyle

Instead of calling Document::styleResolver() constantly inside the loop
we should just keep the resolver on the stack. This saves the constant
null checks inside the loop. They're probably predicted, but even so
it's good to make this code more clear.

I also removed the stale comment about the WhitespaceList since we
deleted all that code.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/68493002

git-svn-id: svn://svn.chromium.org/blink/trunk@161761 bbb929c8-8fbe-4397-9dbb-9b2b20218538