Support trailing 'pxpx' in image size options.

This is a persistent bit of backwards-compatibility.  An attempt was made
to remove this in 2008 which had to be reverted (bug 13500).  And it's
still around today, so Parsoid has to support it.

Bug: 51628
Change-Id: Ia7dda4c075816b0d2873233c366d61db1a304e4a
Merge "Support trailing 'pxpx' in image size options."