Represent speed in GCTracer functions as double instead of int.

test/unittests/heap/gc-tracer-unittest.cc

 // Copyright 2014 the V8 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.
 
 #include <cmath>
 #include <limits>
 
 #include "src/heap/gc-tracer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 19 matching lines @@
   buffer.Push(MakeBytesAndDuration(1, 10000));
   EXPECT_EQ(min_speed,
             GCTracer::AverageSpeed(buffer, MakeBytesAndDuration(0, 0), 1));
   buffer.Reset();
   int sum = 0;
   for (int i = 0; i < buffer.kSize; i++) {
     sum += i + 1;
     buffer.Push(MakeBytesAndDuration(i + 1, 1));
   }
   EXPECT_EQ(
-      static_cast<int>(sum * 1.0 / buffer.kSize + 0.5),
+      sum * 1.0 / buffer.kSize,
       GCTracer::AverageSpeed(buffer, MakeBytesAndDuration(0, 0), buffer.kSize));
   buffer.Push(MakeBytesAndDuration(100, 1));
   EXPECT_EQ(
-      static_cast<int>((sum * 1.0 - 1 + 100) / buffer.kSize + 0.5),
+      (sum * 1.0 - 1 + 100) / buffer.kSize,
       GCTracer::AverageSpeed(buffer, MakeBytesAndDuration(0, 0), buffer.kSize));
 }
 
 }  // namespace internal
 }  // namespace v8