Win64 build fix, change SameSizeAsRootInlineBox to use right types

Change the dummy SameSizeAsRootInlineBox class to use the same field
types as the real class in an effort to make the Win64 compile less
sad.

TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/345843003

git-svn-id: svn://svn.chromium.org/blink/trunk@176568 bbb929c8-8fbe-4397-9dbb-9b2b20218538