Add Log::VPrint and remove duplicated code.

runtime/vm/disassembler.cc

Index: runtime/vm/disassembler.cc
diff --git a/runtime/vm/disassembler.cc b/runtime/vm/disassembler.cc
index fbc102b50f95480e64c9868cb72fdad05a9c6e1d..b74dc7363348df71f5b0cbf57f3ef114423927dd 100644
--- a/runtime/vm/disassembler.cc
+++ b/runtime/vm/disassembler.cc
@@ -33,18 +33,9 @@ void DisassembleToStdout::ConsumeInstruction(char* hex_buffer,
 
 void DisassembleToStdout::Print(const char* format, ...) {
   va_list args;
-
   va_start(args, format);
-  intptr_t len = OS::VSNPrint(NULL, 0, format, args);
+  ISL_VPrint(format, args);
   va_end(args);
-
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
-  va_start(args, format);
-  OS::VSNPrint(p, (len + 1), format, args);
-  va_end(args);
-
-  ISL_Print("%s", p);
-  free(p);
 }