heap: Add default values to UMA metrics struct

Some fields didn't have default values, which made it impossible to
check whether or not all fields are initialized.

Bug: chromium:1056170
Change-Id: I8c6f89e64661420460bfc351473a47a623ef6399
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3243462Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77535}

include/v8-metrics.h

@@ -46,12 +46,12 @@ struct GarbageCollectionFullCycle {
   GarbageCollectionSizes objects_cpp;
   GarbageCollectionSizes memory;
   GarbageCollectionSizes memory_cpp;
-  double collection_rate_in_percent;
-  double collection_rate_cpp_in_percent;
-  double efficiency_in_bytes_per_us;
-  double efficiency_cpp_in_bytes_per_us;
-  double main_thread_efficiency_in_bytes_per_us;
-  double main_thread_efficiency_cpp_in_bytes_per_us;
+  double collection_rate_in_percent = -1.0;
+  double collection_rate_cpp_in_percent = -1.0;
+  double efficiency_in_bytes_per_us = -1.0;
+  double efficiency_cpp_in_bytes_per_us = -1.0;
+  double main_thread_efficiency_in_bytes_per_us = -1.0;
+  double main_thread_efficiency_cpp_in_bytes_per_us = -1.0;
 };
 
 struct GarbageCollectionFullMainThreadIncrementalMark {