Add some asserts and explict setting of fIsMipMapped

Part 2 of 3 for relanding of https://skia-review.googlesource.com/c/skia/+/42083

Bug: skia:
Change-Id: Iddc2571d88486531c76ab47432c2a51f2ac79043
Reviewed-on: https://skia-review.googlesource.com/44463
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
7 files changed