Sync with upstream repo.

Changes include:
* CPU check has been broken up into a number of small libraries
* BoringSSL option has been removed
* Better abseil integration

stopper.h

@@ -0,0 +1,58 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef THIRD_PARTY_CPU_CHECK_STOPPER_H_
+#define THIRD_PARTY_CPU_CHECK_STOPPER_H_
+
+#include <unistd.h>
+
+#include <atomic>
+#include <cmath>
+
+#include "utils.h"
+
+class Stopper {
+ public:
+  // Infinite timeout if 'timeout' <= 0.
+  Stopper(int timeout)
+      : t_stop_(timeout <= 0 ? std::numeric_limits<double>::infinity()
+                             : TimeInSeconds() + timeout) {}
+
+  // Returns true if time has expired or Stop has been invoked.
+  // Thread safe.
+  bool Expired() const { return stopped_ || TimeInSeconds() > t_stop_; }
+
+  // Sleeps for the minimum of 't' and remaining run time.
+  // Thread safe.
+  void BoundedSleep(int t) const {
+    if (std::isinf(t_stop_)) {
+      sleep(t);
+    } else {
+      const double remaining = t_stop_ - TimeInSeconds();
+      if (!stopped_ && remaining > 0) {
+        sleep(std::min<int>(t, ceil(remaining)));
+      }
+    }
+  }
+
+  // Causes timeout to expire now.
+  // Thread safe.
+  void Stop() { stopped_ = true; }
+
+ private:
+  const double t_stop_;
+  std::atomic_bool stopped_ = false;
+};
+
+#endif  // THIRD_PARTY_CPU_CHECK_STOPPER_H_