Prettyprinting for layout units

This will lead to gdb output like:

        m_frameRect = {
          m_location = LayoutPoint(0px, 0px),
          m_size = LayoutSize(800px, 585px)
        },
        m_marginBox = {
          m_top = 0px,
          m_right = 0px,
          m_bottom = 0px,
          m_left = 0px
        },
        m_minPreferredLogicalWidth = 0px,
        m_maxPreferredLogicalWidth = 0px,
        m_intrinsicContentLogicalHeight = 585px,

Much easier to read than having to divide by 64 in your head!

Non-integer pixels will print like:
  m_location = LayoutPoint(0.5px x 0px),

BUG=
R=tony@chromium.org, abarth@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/118453004

git-svn-id: svn://svn.chromium.org/blink/trunk@164115 bbb929c8-8fbe-4397-9dbb-9b2b20218538