win: avoid warning on return of base::RandGenerator
win: avoid warning on return of base::RandGenerator

Else,

d:\src\crashpad\crashpad\util\net\http_multipart_builder.cc(50) : warning C4244: 'initializing' : conversion from 'uint64_t' to 'int', possible loss of data

R=mark@chromium.org
BUG=crashpad:1

Review URL: https://codereview.chromium.org/796643006

chromium/crashpad,atom/crashpad,Chilledheart/crashpad,atom/crashpad,atom/crashpad,hankbao/Crashpad,Chilledheart/crashpad,chromium/crashpad,Chilledheart/crashpad,hokein/crashpad,hankbao/Crashpad,hankbao/Crashpad,chromium/crashpad,hokein/crashpad,hokein/crashpad