[GCM] Don't attempt to run a null callback.

[GCM] Don't attempt to run a null callback.

BUG=none
TEST=none
R=jianli@chromium.org

[Ilya Sherman, 2014-05-09 03:51:19]

I don't know whether this is the correct fix, but I observed this callback being
null in my local build.  Either there should be a null-check as I've added here,
or there is some intended invariant that is not being maintained.

[jianli, 2014-05-09 20:05:39]

Thanks for looking into this. I think this is covered in this patch that is
being reviewed: https://codereview.chromium.org/277743002/.

[Ilya Sherman, 2014-05-09 20:10:43]

Thanks.  Closing this, then.