Make standalone D8 trace to single hydrogen.cfg file.

src/hydrogen.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2067 matching lines @@
   HGraph* graph_;
 
   DISALLOW_COPY_AND_ASSIGN(HPhase);
 };
 
 
 class HTracer: public Malloced {
  public:
   explicit HTracer(int isolate_id)
       : trace_(&string_allocator_), indent_(0) {
-    OS::SNPrintF(filename_,
-                 "hydrogen-%d-%d.cfg",
-                 OS::GetCurrentProcessId(),
-                 isolate_id);
+    if (FLAG_trace_hydrogen_file == NULL) {
+      OS::SNPrintF(filename_,
+                   "hydrogen-%d-%d.cfg",
+                   OS::GetCurrentProcessId(),
+                   isolate_id);
+    } else {
+      OS::StrNCpy(filename_, FLAG_trace_hydrogen_file, filename_.length());
+    }
     WriteChars(filename_.start(), "", 0, false);
   }
 
   void TraceCompilation(CompilationInfo* info);
   void TraceHydrogen(const char* name, HGraph* graph);
   void TraceLithium(const char* name, LChunk* chunk);
   void TraceLiveRanges(const char* name, LAllocator* allocator);
 
  private:
   class Tag BASE_EMBEDDED {
@@ skipping 57 matching lines @@
   EmbeddedVector<char, 64> filename_;
   HeapStringAllocator string_allocator_;
   StringStream trace_;
   int indent_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_