eliminate use of Usage in finalize

It's just as easy to calculate the death time directly as it is to use
Usage, and it saves some allocations and a little CPU time we don't need
to spend.

Left Usage in place in schedule() for you,
but did leave in the slight tweak to the assert.

Change-Id: I285833e3aaf855e7bc3cc7cf1ff826a5c01313e2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281235
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed