Move CompilerStats from isolate to thread. Aggregate stats.

runtime/vm/timer.h

Index: runtime/vm/timer.h
diff --git a/runtime/vm/timer.h b/runtime/vm/timer.h
index f3b15fafaa1013fc160b76a00a2446fce467738d..01a6b6fe62d1722866e5a2b5107e84e1331ea781 100644
--- a/runtime/vm/timer.h
+++ b/runtime/vm/timer.h
@@ -7,6 +7,7 @@
 
 #include "platform/utils.h"
 #include "vm/allocation.h"
+#include "vm/atomic.h"
 #include "vm/flags.h"
 #include "vm/os.h"
 
@@ -34,7 +35,8 @@ class Timer : public ValueObject {
     stop_ = OS::GetCurrentTimeMicros();
     int64_t elapsed = ElapsedMicros();
     max_contiguous_ = Utils::Maximum(max_contiguous_, elapsed);
-    total_ += elapsed;
+    // Make increment atomic in case it occurs in parallel with aggregation.
+    AtomicOperations::IncrementInt64By(&total_, elapsed);
     running_ = false;
   }
 
@@ -65,6 +67,15 @@ class Timer : public ValueObject {
     running_ = false;
   }
 
+  bool IsReset() const {
+    return (start_ == 0) && (stop_ == 0) && (total_ == 0) &&
+        (max_contiguous_ == 0) && !running_;
+  }
+
+  void AddTotal(const Timer& other) {
+    AtomicOperations::IncrementInt64By(&total_, other.total_);
+  }
+
   // Accessors.
   bool report() const { return report_; }
   bool running() const { return running_; }