[fuchsia] Bypass memory size requirements for sysmem-backed images.

For sysmem-backed images, we bypass the memory size requirements,
instead we use the sysmem-allocated-size (currently it's width *
height * 4) instead.

This fixes web-engine crash on FEMU.

TEST: 1) fx shell tiles_ctl start
      2) fx shell tiles_ctl add "http://www.google.com/"

Change-Id: Ia2aa50f966f9888b5a2f531e2023d8ed09f2a288
1 file changed