Add OS::GetCurrentProcessId and prepend output from trace-gc with the current pid

test/cctest/test-platform-linux.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 //
 // Tests of the TokenLock class from lock.h
 
 #include <pthread.h>
 #include <stdlib.h>
 #include <unistd.h>  // for usleep()
 
 #include "v8.h"
 
@@ skipping 61 matching lines @@
   CHECK(vm->IsReserved());
   void* block_addr = vm->address();
   size_t block_size = 4 * KB;
   CHECK(vm->Commit(block_addr, block_size, false));
   // Check whether we can write to memory.
   int* addr = static_cast<int*>(block_addr);
   addr[KB-1] = 2;
   CHECK(vm->Uncommit(block_addr, block_size));
   delete vm;
 }
+
+TEST(GetCurrentProcessId) {
+  OS::SetUp();
+  CHECK_EQ(static_cast<int>(getpid()), OS::GetCurrentProcessId());
+}