Expose new GC types introduced in v8 version 4.6

Everything that was not kGCTypeMarkSweepCompact used to be mapped to "S"
(Scavenge), but that became wrong with 4.6, which has more GC types.

Author: sam-github

README.md

@@ -238,7 +238,13 @@ Emitted every 5 seconds, summarising sample based information of the event loop
 Emitted when a garbage collection (GC) cycle occurs in the underlying V8 runtime.
 * `data` (Object) the data from the GC sample:
     * `time` (Number) the milliseconds when the sample was taken. This can be converted to a Date using `new Date(data.time)`.
-    * `type` (String) the type of GC cycle, either 'M' or 'S'.
+    * `type` (String) the type of GC cycle, either:
+      - `'M'`: MarkSweepCompact, aka "major"
+      - `'S'`: Scavenge, aka "minor"
+      - `'I'`: IncrementalMarking, aka "incremental" (only exists on node 5.x
+        and greater)
+      - '`W'`: ProcessWeakCallbacks, aka "weakcb" (only exists on node 5.x
+        and greater)
     * `size` (Number) the size of the JavaScript heap in bytes.
     * `used` (Number) the amount of memory used on the JavaScript heap in bytes.
     * `duration` (Number) the duration of the GC cycle in milliseconds.

src/plugins/node/gc/nodegcplugin.cpp

@@ -149,7 +149,17 @@ void afterGC(v8::Isolate *isolate, GCType type, GCCallbackFlags flags) {
 	gcRealEnd = GetRealTime();
 
 	// GC type
-	const char *gcType = (type == kGCTypeMarkSweepCompact) ? "M" : "S";
+	const char *gcType = NULL;
+        switch (type) {
+          case kGCTypeMarkSweepCompact: gcType = "M"; break;
+          case kGCTypeScavenge: gcType = "S"; break;
+#if NODE_VERSION_AT_LEAST(5, 0, 0)
+          case kGCTypeIncrementalMarking: gcType = "I"; break;
+          case kGCTypeProcessWeakCallbacks: gcType = "W"; break;
+#endif
+          // Should never happen, but call it minor if type is unrecognized.
+          default: gcType = "S"; break;
+        }
 
 	// GC heap stats
 	HeapStatistics hs;