The compiler can complain about these signed to unsigned conversions.
Explicitly cast them in initWithPreallocatedStorage to match the way
this is already done in init [0].
[0] 311b648013 "pack SkTArray"
Change-Id: I0f723094fd356f9c5971b35998c4ecd59c09332f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/536099
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>