use experimental ubsan config

bazel/grpc_deps.bzl

@@ -176,12 +176,11 @@ def grpc_deps():
     if "bazel_toolchains" not in native.existing_rules():
         http_archive(
             name = "bazel_toolchains",
-            strip_prefix = "bazel-toolchains-d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4",
+            strip_prefix = "bazel-toolchains-68350603967ef8923b02d9644e505c75a11cad4f",
             urls = [
-                "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4.tar.gz",
+                "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/68350603967ef8923b02d9644e505c75a11cad4f.tar.gz",
-                "https://github.com/bazelbuild/bazel-toolchains/archive/d665ccfa3e9c90fa789671bf4ef5f7c19c5715c4.tar.gz",
             ],
-            sha256 = "4b1468b254a572dbe134cc1fd7c6eab1618a72acd339749ea343bd8f55c3b7eb",
+            sha256 = "47eab1412df2df0de26cf9d017954302d2dd93b324646b24706a8c5f19dd27df",
         )
 
     if "bazel_skylib" not in native.existing_rules():

tools/remote_build/rbe_common.bazelrc

@@ -84,4 +84,4 @@ build:ubsan --copt=-gmlt
 # TODO(jtattermusch): use more reasonable test timeout
 build:ubsan --test_timeout=3600
 # override the config-agnostic crosstool_top
-build:ubsan --crosstool_top=@rbe_ubsan//cc:toolchain
+build:ubsan --crosstool_top=@com_github_bazelbuild_bazeltoolchains//configs/experimental/ubuntu16_04_clang/1.2/bazel_0.21.0/ubsan:toolchain