Merge github.com:grpc/grpc into slice_with_exec_ctx

BUILD

CMakeLists.txt

Makefile

build.yaml

gRPC-Core.podspec

grpc.gemspec

include/grpc/impl/codegen/gpr_slice.h

include/grpc/impl/codegen/slice.h

package.xml

src/core/ext/client_channel/client_channel.c

src/core/lib/support/string.h

src/core/lib/support/tmpfile.h

test/core/end2end/end2end_nosec_tests.c

test/core/end2end/end2end_tests.c

test/core/end2end/gen_build_yaml.py

test/core/end2end/tests/short_deadlines.c

test/core/surface/public_headers_must_be_c89.c

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/doxygen/Doxyfile.core