Make size_t -> unsigned int conversion explicit
Make size_t -> unsigned int conversion explicit

This fixes MSVC compiler warnings.

Change-Id: Iadd603a4ce2f944f708dff6b23005991cfa3d47a

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom