Cap id size returned by GrScratchKey::GenerateResourceType

Cap id size returned by GrScratchKey::GenerateResourceType

kkinnunen@ has been seeing compiler warnings on GrScratchKey::GenerateResourceType (https://codereview.chromium.org/833913002/ - Cast the scratch resource key in order to fix a warning)(presumably due to the unsigned/signed comparison in kInvalidResourceType == type). This CL limits the returned id to its effective range to, hopefully, clarify the whole int32_t -> uint32_t conversion & wrapping problem.

Review URL: https://codereview.chromium.org/827923006