Don't shift an int left while assigning it to a size_t

Don't shift an int left while assigning it to a size_t

Summary:
MSVC gives warnings if you shift a 32-bit value left but then assign it to a 64-bit variable. This just makes it a 64-bit shift instead.
If we really wanted, this doesn't need to be a size_t to begin with, but it already is, so just leave it alone.

Reviewed By: yfeldblum

Differential Revision: D3622935

fbshipit-source-id: 25931e6df644df8a2160aa80fd5de21cd9c06159