Report the reason for a garbage collection when verbose GC is enabled.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 8db0aefa35f987eb2239b93dc6c5123d3e21a0e3..f803a85d362626b1c546a8837b5d6fa3e3fd6cb9 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -43,7 +43,7 @@ class Scavenger {
     ASSERT(Utils::IsAligned(size, kObjectAlignment));
 #if defined(DEBUG)
     if (FLAG_gc_at_alloc && !scavenging_) {
-      Scavenge();
+      Scavenge("debugging");
     }
 #endif
     uword result = top_;
@@ -60,8 +60,8 @@ class Scavenger {
   }
 
   // Collect the garbage in this scavenger.
-  void Scavenge();
-  void Scavenge(bool invoke_api_callbacks);
+  void Scavenge(const char* gc_reason);
+  void Scavenge(bool invoke_api_callbacks, const char* gc_reason);
 
   // Accessors to generate code for inlined allocation.
   uword* TopAddress() { return &top_; }