Make lldb pretty print LayoutUnit, LayoutSize, and LayoutPoint

You end up getting something like:

    (lldb) p floatingObject->frameRect()
    (const WebCore::LayoutRect) $0 = {
      m_location = { x = 744px, y = 1px }
      m_size = { width = 236px, height = 40px }
    }

This is based off of cbiesinger's patch for the gdb pretty printers,
though I changed the format a bit to try and stay with the format used
by the existing lldb pretty printers.

Review URL: https://codereview.chromium.org/106023005

git-svn-id: svn://svn.chromium.org/blink/trunk@164564 bbb929c8-8fbe-4397-9dbb-9b2b20218538