Merge branch 'google3' to roll forward up to cl/353292511.

cpp/platform/base/BUILD

@@ -94,6 +94,28 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "cancellation_flag",
+    srcs = [
+        "cancellation_flag.cc",
+    ],
+    hdrs = [
+        "cancellation_flag.h",
+    ],
+    visibility = [
+        "//core/internal:__subpackages__",
+        "//platform/api:__subpackages__",
+        "//platform/impl:__subpackages__",
+        "//platform/public:__pkg__",
+    ],
+    deps = [
+        ":base",
+        ":util",
+        "//absl/container:flat_hash_set",
+        "//absl/synchronization",
+    ],
+)
+
 cc_library(
     name = "test_util",
     testonly = True,
@@ -143,6 +165,17 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "cancellation_flag_test",
+    srcs = [
+        "cancellation_flag_test.cc",
+    ],
+    deps = [
+        ":cancellation_flag",
+        "//testing/base/public:gunit_main",
+    ],
+)
+
 cc_with_non_compile_test(
     name = "exception_test",
     srcs = [