Merged changes from upstream/master

BUILD

Makefile

build.json

gRPC.podspec

grpc.bzl

include/grpc++/client_context.h

include/grpc/census.h

include/grpc/grpc.h

include/grpc/support/atm_win32.h

include/grpc/support/log_win32.h

include/grpc/support/port_platform.h

include/grpc/support/sync_win32.h

src/core/census/grpc_context.c

src/core/census/grpc_context.h

src/core/census/initialize.c

src/core/channel/census_filter.c

src/core/channel/channel_args.c

src/core/channel/channel_args.h

src/core/channel/channel_stack.c

src/core/channel/channel_stack.h