set maximum image size

Our use of web settings does not specify the maximum image size,
although we have some hard-coded values used when decoding the
image. For defective images, the hard-coded values are not enough,
since we may exceed memory in the decoder before the limit is
reached.

Pass along our limits to the decoder, and respect any limits
set by the caller.

companion change in external/webkit

Change-Id: Iabd78165390dddce917f637bd3263b83f9b4aa56
http://b/2689053