pack SkTArray
pack SkTArray

Same idea as http://review.skia.org/325857; I just wanted to feel out
the options myself. A couple key ideas:

    Prefer SkTo32() to separate range assertions and casts.

    Keep using int indices, counting to this->count() where
    fCount would have warned about signed/unsigned mismatch.

I've kept new comments and assertions to minimum.  In the end we won't
change the max size of SkTArray, and I don't see much need to call out
how much it hasn't changed.  Reading back over the accumulated changes,
I don't really see much that's newly error-prone.

Change-Id: I86a8a161b9ae44f24fc25093741945b75fbfa770
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326106
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia