Rewrote the PRNG using an xorshift* algorithm and moved the files from test/ to base/.

Created a simple unit test for the new random number generator. (It mostly tests
that the generated numbers are consistent with the intended distribution, e.g. uniform.
It is not a comprehensive test of the quality of the random numbers.)

Several assertions in OveruseDetectorTest seem to depend on the exact sequence of random numbers. I updated those numbers to work with the new PRNG.

Compute the standard deviation of the expected result in TestReorderFilter instead of passing an uncertainty parameter.

BUG=webrtc:5177

Review URL: https://codereview.webrtc.org/1457023002

Cr-Commit-Position: refs/heads/master@{#10965}

webrtc/base/random.h

@@ -0,0 +1,82 @@
+/*
+ *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_RANDOM_H_
+#define WEBRTC_BASE_RANDOM_H_
+
+#include <limits>
+
+#include "webrtc/typedefs.h"
+#include "webrtc/base/constructormagic.h"
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+class Random {
+ public:
+  explicit Random(uint64_t seed);
+
+  // Return pseudo-random integer of the specified type.
+  // We need to limit the size to 32 bits to keep the output close to uniform.
+  template <typename T>
+  T Rand() {
+    static_assert(std::numeric_limits<T>::is_integer &&
+                      std::numeric_limits<T>::radix == 2 &&
+                      std::numeric_limits<T>::digits <= 32,
+                  "Rand is only supported for built-in integer types that are "
+                  "32 bits or smaller.");
+    return static_cast<T>(NextOutput());
+  }
+
+  // Uniformly distributed pseudo-random number in the interval [0, t].
+  uint32_t Rand(uint32_t t);
+
+  // Uniformly distributed pseudo-random number in the interval [low, high].
+  uint32_t Rand(uint32_t low, uint32_t high);
+
+  // Uniformly distributed pseudo-random number in the interval [low, high].
+  int32_t Rand(int32_t low, int32_t high);
+
+  // Normal Distribution.
+  double Gaussian(double mean, double standard_deviation);
+
+  // Exponential Distribution.
+  double Exponential(double lambda);
+
+ private:
+  // Outputs a nonzero 64-bit random number.
+  uint64_t NextOutput() {
+    state_ ^= state_ >> 12;
+    state_ ^= state_ << 25;
+    state_ ^= state_ >> 27;
+    RTC_DCHECK(state_ != 0x0ULL);
+    return state_ * 2685821657736338717ull;
+  }
+
+  uint64_t state_;
+
+  RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(Random);
+};
+
+// Return pseudo-random number in the interval [0.0, 1.0).
+template <>
+float Random::Rand<float>();
+
+// Return pseudo-random number in the interval [0.0, 1.0).
+template <>
+double Random::Rand<double>();
+
+// Return pseudo-random boolean value.
+template <>
+bool Random::Rand<bool>();
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_BASE_RANDOM_H_