Bug 1233831 - Part 1: Call the callback on nursery collections; r=terrence

This commit adds the `js::gcstats::Statistics::{begin,end}NurseryCollection`
methods which calls the nursery collection callback, if present, and
additionally handles counting minor GCs (which used to be inline in
js::Nursery::collect).

js/src/gc/Nursery.cpp

@@ -418,8 +418,7 @@ js::Nursery::collect(JSRuntime* rt, JS::gcreason::Reason reason, ObjectGroupList
 
     rt->gc.incMinorGcNumber();
 
-    rt->gc.stats.count(gcstats::STAT_MINOR_GC);
-
+    rt->gc.stats.beginNurseryCollection(reason);
     TraceMinorGCStart();
 
     int64_t timestampStart_total = PRMJ_Now();
@@ -563,6 +562,7 @@ js::Nursery::collect(JSRuntime* rt, JS::gcreason::Reason reason, ObjectGroupList
     if (totalTime > 1000)
         rt->addTelemetry(JS_TELEMETRY_GC_MINOR_REASON_LONG, reason);
 
+    rt->gc.stats.endNurseryCollection(reason);
     TraceMinorGCEnd();
 
     if (enableProfiling_ && totalTime >= profileThreshold_) {

js/src/gc/Statistics.h

@@ -191,6 +191,23 @@ struct Statistics
         counts[s]++;
     }
 
+    void beginNurseryCollection(JS::gcreason::Reason reason) {
+        count(STAT_MINOR_GC);
+        if (nurseryCollectionCallback) {
+            (*nurseryCollectionCallback)(runtime,
+                                         JS::GCNurseryProgress::GC_NURSERY_COLLECTION_START,
+                                         reason);
+        }
+    }
+
+    void endNurseryCollection(JS::gcreason::Reason reason) {
+        if (nurseryCollectionCallback) {
+            (*nurseryCollectionCallback)(runtime,
+                                         JS::GCNurseryProgress::GC_NURSERY_COLLECTION_END,
+                                         reason);
+        }
+    }
+
     int64_t beginSCC();
     void endSCC(unsigned scc, int64_t start);