Fix crashes while displaying large animated GIFs. Our old understanding of t...

Fix crashes while displaying large animated GIFs.  Our old understanding of the memory model here was wrong.  This moves to the Cairo way of doing things.  (An alternative would be to change NativeImagePtr from a raw pointer to a ref-counting pointer, like the Mac does.  This would ape the Mac API, which is perhaps more future-proof, at the cost of needless refcounting complexity [since SkBitmap internally refs its pixel data already].)

BUG=4298
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=5860

[brettw, 2008-11-21 22:46:22]

LGTM