Fix Pepper URL Loader callbacks.

Fix Pepper URL Loader callbacks.

(Previously committed r71334, reverted r71400. Added null check pointed out by
jam.)

Add some tests (for aborting calls).

BUG=none
TEST=ppapi_tests

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=71463

[viettrungluu, 2011-01-14 01:31:23]

This is just what I had previously committed (and then reverted), with your
additional fix (+ a CHECK(), to make sure that nothing strange happens).

[jam, 2011-01-14 02:06:30]

lgtm