Delete odd assignment of IgnoreOverlayScrollbarSize

This code doesn't many any sense. I also added a RELEASE_ASSERT that the field
already had this value and every test passed. That means this code doesn't do
anything.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/345463010

git-svn-id: svn://svn.chromium.org/blink/trunk@177156 bbb929c8-8fbe-4397-9dbb-9b2b20218538