Bug 1233831 - Part 2: Expose JS::gcreason::ExplainReason to embedders; r=terrence

This renames the internal function `js::gcstats::ExplainReason` to
`JS::gcreason::ExplainReason` and publicly exposes it to embedders.

js/public/GCAPI.h

@@ -128,6 +128,12 @@ enum Reason {
     NUM_TELEMETRY_REASONS = 100
 };
 
+/**
+ * Get a statically allocated C string explaining the given GC reason.
+ */
+extern JS_PUBLIC_API(const char*)
+ExplainReason(JS::gcreason::Reason reason);
+
 } /* namespace gcreason */
 
 /*

js/src/gc/Nursery.cpp

@@ -600,7 +600,7 @@ js::Nursery::collect(JSRuntime* rt, JS::gcreason::Reason reason, ObjectGroupList
         }
 
 #define FMT " %6" PRIu64
-        fprintf(stderr, "MinorGC: %20s %5.1f%% %4d " FMT, js::gcstats::ExplainReason(reason),
+        fprintf(stderr, "MinorGC: %20s %5.1f%% %4d " FMT, JS::gcreason::ExplainReason(reason),
                 promotionRate * 100, numActiveChunks_, totalTime);
         for (auto &entry : PrintList) {
             fprintf(stderr, FMT, entry.time);

js/src/gc/Statistics.cpp

@@ -48,8 +48,8 @@ js::gcstats::ExplainInvocationKind(JSGCInvocationKind gckind)
          return "Shrinking";
 }
 
-const char*
-js::gcstats::ExplainReason(JS::gcreason::Reason reason)
+JS_PUBLIC_API(const char*)
+JS::gcreason::ExplainReason(JS::gcreason::Reason reason)
 {
     switch (reason) {
 #define SWITCH_REASON(name)                         \

js/src/gc/Statistics.h

@@ -423,7 +423,6 @@ struct MOZ_RAII AutoSCC
 };
 
 const char* ExplainInvocationKind(JSGCInvocationKind gckind);
-const char* ExplainReason(JS::gcreason::Reason reason);
 
 } /* namespace gcstats */
 } /* namespace js */

js/src/vm/Debugger.cpp

@@ -8519,7 +8519,7 @@ GarbageCollectionEvent::Create(JSRuntime* rt, ::js::gcstats::Statistics& stats,
             // reasons this data is stored and replicated on each slice. Each
             // slice used to have its own GCReason, but now they are all the
             // same.
-            data->reason = gcstats::ExplainReason(range.front().reason);
+            data->reason = gcreason::ExplainReason(range.front().reason);
             MOZ_ASSERT(data->reason);
         }