Adds arithmetic support to DataRate.

Bug: webrtc:9709
Change-Id: Id3fde2b7bbc63c6372cadc76b92c4568665b17f9
Reviewed-on: https://webrtc-review.googlesource.com/97702
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24562}

api/units/data_rate.h

@@ -133,7 +133,20 @@ class DataRate {
     return bits_per_sec_ == data_rate_impl::kPlusInfinityVal;
   }
   constexpr bool IsFinite() const { return !IsInfinite(); }
-
+  DataRate operator-(const DataRate& other) const {
+    return DataRate::bps(bps() - other.bps());
+  }
+  DataRate operator+(const DataRate& other) const {
+    return DataRate::bps(bps() + other.bps());
+  }
+  DataRate& operator-=(const DataRate& other) {
+    *this = *this - other;
+    return *this;
+  }
+  DataRate& operator+=(const DataRate& other) {
+    *this = *this + other;
+    return *this;
+  }
   constexpr double operator/(const DataRate& other) const {
     return bps<double>() / other.bps<double>();
   }

api/units/data_rate_unittest.cc

@@ -100,11 +100,21 @@ TEST(DataRateTest, MathOperations) {
   const DataRate rate_b = DataRate::bps(kValueB);
   const int32_t kInt32Value = 123;
   const double kFloatValue = 123.0;
+
+  EXPECT_EQ((rate_a + rate_b).bps(), kValueA + kValueB);
+  EXPECT_EQ((rate_a - rate_b).bps(), kValueA - kValueB);
+
   EXPECT_EQ((rate_a * kValueB).bps(), kValueA * kValueB);
   EXPECT_EQ((rate_a * kInt32Value).bps(), kValueA * kInt32Value);
   EXPECT_EQ((rate_a * kFloatValue).bps(), kValueA * kFloatValue);
 
   EXPECT_EQ(rate_a / rate_b, static_cast<double>(kValueA) / kValueB);
+
+  DataRate mutable_rate = DataRate::bps(kValueA);
+  mutable_rate += rate_b;
+  EXPECT_EQ(mutable_rate.bps(), kValueA + kValueB);
+  mutable_rate -= rate_a;
+  EXPECT_EQ(mutable_rate.bps(), kValueB);
 }
 
 TEST(UnitConversionTest, DataRateAndDataSizeAndTimeDelta) {

api/units/data_size.h

@@ -95,11 +95,11 @@ class DataSize {
     return DataSize::bytes(bytes() + other.bytes());
   }
   DataSize& operator-=(const DataSize& other) {
-    bytes_ -= other.bytes();
+    *this = *this - other;
     return *this;
   }
   DataSize& operator+=(const DataSize& other) {
-    bytes_ += other.bytes();
+    *this = *this + other;
     return *this;
   }
   constexpr double operator/(const DataSize& other) const {