Fix handling of block with left margin next to a float

When refactoring the code, I ended up adding the margin in instead of
subtracting it in the case of an intruding float. This patch fixes that
so that the margin is properly handled.

BUG=294486

Review URL: https://chromiumcodereview.appspot.com/24119003

git-svn-id: svn://svn.chromium.org/blink/trunk@157994 bbb929c8-8fbe-4397-9dbb-9b2b20218538