Fix google#382 - MinGW often reports negative CPU times. (google#475)

When stopping a timer, the current time is subtracted
from the start time. However, when the times are identical,
or sufficiently close together, the subtraction can result
in a negative number.

For some reason MinGW is the only platform where this problem
manifests. I suspect it's due to MinGW specific behavior in either
the CPU timing code, floating point model, or printf formatting.

Either way, the fix for MinGW should be correct across all platforms.

src/benchmark.cc

@@ -175,7 +175,9 @@ class ThreadTimer {
     CHECK(running_);
     running_ = false;
     real_time_used_ += ChronoClockNow() - start_real_time_;
-    cpu_time_used_ += ThreadCPUUsage() - start_cpu_time_;
+    // Floating point error can result in the subtraction producing a negative
+    // time. Guard against that.
+    cpu_time_used_ += std::max<double>(ThreadCPUUsage() - start_cpu_time_, 0);
   }
 
   // Called by each thread