Initial support for emitting unwinding information in perf jitdump.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index d03b1a8e6035609b3485ff64957f6616cb103292..3345e30538f53924348848a92778926a93fb1b4a 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -320,6 +320,8 @@ void Assembler::GetCode(CodeDesc* desc) {
       static_cast<int>((buffer_ + buffer_size_) - reloc_info_writer.pos());
   desc->origin = this;
   desc->constant_pool_size = 0;
+  desc->unwinding_info_size = 0;
+  desc->unwinding_info = nullptr;
 }