[ObjC]fix 64 to 32 bit clang conversion warning in src/core/lib/transport

[HannahShiSFB]

https://github.com/grpc/grpc-ios/issues/83

fix 64 to 32 bit clang conversion warning in src/core/lib/transport

@dennycd

[ctiller]

Will there be a test in place once this is done to ensure no regressions?

[HannahShiSFB]

Will there be a test in place once this is done to ensure no regressions?

We plan to make these warnings as errors after these are done so regressions will break the build