[heap] Initialize Heap::total_gc_time_ms_

Change-Id: I5f73a541d22257d4fbb21e619ad2b62068c267f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879940Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64565}

src/heap/heap.h

@@ -1979,7 +1979,7 @@ class Heap {
   unsigned int maximum_size_scavenges_ = 0;
 
   // Total time spent in GC.
-  double total_gc_time_ms_;
+  double total_gc_time_ms_ = 0.0;
 
   // Last time an idle notification happened.
   double last_idle_notification_time_ = 0.0;