chakrashim: Move unordered_set to chakrashim

* Moved `unordered_set` to `v8-tracing.h` to match v8
* Fixed `int64`->`double` implicit cast in `v8-platform.h`

PR-URL: #466
Reviewed-By: Taylor Woll <tawoll@ntdev.microsoft.com>
Reviewed-By: Hitesh Kanwathirtha <hiteshk@microsoft.com>

deps/chakrashim/include/libplatform/v8-tracing.h

@@ -7,7 +7,9 @@
 
 #include <fstream>
 #include <memory>
+#include <unordered_set>
 #include <vector>
+
 #include "v8-platform.h"
 
 namespace v8 {

deps/chakrashim/include/v8-platform.h

@@ -202,8 +202,9 @@ class Platform {
 
  protected:
   static double SystemClockTimeMillis() {
-    return std::chrono::duration_cast<std::chrono::milliseconds>(
-        std::chrono::system_clock::now().time_since_epoch()).count();
+    return std::chrono::duration_cast<
+        std::chrono::duration<double, std::milli>>(
+            std::chrono::system_clock::now().time_since_epoch()).count();
   }
 };