Fix computation of texture size for approximately-fit deferred proxies

Fix computation of texture size for approximately-fit deferred proxies

For approximate-fit deferred proxies asserts were firing when the instantiated size was larger than the pre-computed exact-fit size.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4972

Change-Id: I879e16b86ab4d9ef9834163c24ccd6507fe4b94a
Reviewed-on: https://skia-review.googlesource.com/4972
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>