Make mask uintptr_t when calculating alignment.
Make mask uintptr_t when calculating alignment.

A suggestion from from the code review on cl/7614.

TBR=mtklein@google.com

Change-Id: Icfbe08bfe63ac1e21ed56481dc5251c2af5bb96a
Reviewed-on: https://skia-review.googlesource.com/8674
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

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