Fix test-cpu-profiler/FunctionCallSample flakiness under GC stress testing

test/cctest/test-cpu-profiler.cc

 // Copyright 2010 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 1069 matching lines @@
   {
     ScopedVector<v8::Handle<v8::String> > names(4);
     names[0] = v8::String::New(ProfileGenerator::kGarbageCollectorEntryName);
     names[1] = v8::String::New(ProfileGenerator::kProgramEntryName);
     names[2] = v8::String::New("start");
     names[3] = v8::String::New(i::ProfileGenerator::kUnresolvedFunctionName);
     // Don't allow |bar| and |call| nodes to be at the top level.
     CheckChildrenNames(root, names);
   }
 
-  const v8::CpuProfileNode* startNode = GetChild(root, "start");
-  {
+  // In case of GC stress tests all samples may be in GC phase and there
+  // won't be |start| node in the profiles.
+  bool start_node_must_exist =
+      (i::FLAG_gc_interval == -1) && !i::FLAG_stress_compaction;
+  const v8::CpuProfileNode* startNode = start_node_must_exist
+                                      ? GetChild(root, "start")
+                                      : FindChild(root, "start");
+  if (startNode) {
     ScopedVector<v8::Handle<v8::String> > names(1);
     names[0] = v8::String::New("bar");
     CheckChildrenNames(startNode, names);
   }
 
   const v8::CpuProfileNode* unresolvedNode =
       FindChild(root, i::ProfileGenerator::kUnresolvedFunctionName);
   if (unresolvedNode) {
     ScopedVector<v8::Handle<v8::String> > names(1);
     names[0] = v8::String::New("call");
     CheckChildrenNames(unresolvedNode, names);
   }
 
   cpu_profiler->DeleteAllCpuProfiles();
 }