remove off64_t in SkHeifCodec

I argue int64_t is just as good, and much more familiar.

This, I think, will also fix the Emscripten/wasm bot,
where somewhat bizarrely off64_t is typedef'd to int.

Change-Id: I4eb9d05cbd3066b107fa429beff79e7471e94526
Reviewed-on: https://skia-review.googlesource.com/62362
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed