Is char* or uintptr_t easier to read?

Using a char* instead of uintptr_t allows us to use fewer
reinterpret_casts which may make the code easier to read.

BUG=648512
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2356703002

Review-Url: https://codereview.chromium.org/2356703002