win: Retrieve thread context for x64

snapshot/win/thread_snapshot_win.h

Index: snapshot/win/thread_snapshot_win.h
diff --git a/snapshot/win/thread_snapshot_win.h b/snapshot/win/thread_snapshot_win.h
index 19e8066c22c801283bf3ef88b51993e0198c2b65..9829707f631ffc7bf0a018274bf1d33dc5a8c43a 100644
--- a/snapshot/win/thread_snapshot_win.h
+++ b/snapshot/win/thread_snapshot_win.h
@@ -60,6 +60,12 @@ class ThreadSnapshotWin final : public ThreadSnapshot {
   uint64_t ThreadSpecificDataAddress() const override;
 
  private:
+#if defined(ARCH_CPU_X86_FAMILY)
+  union {
+    CPUContextX86 x86;
+    CPUContextX86_64 x86_64;
+  } context_union_;
+#endif
   CPUContext context_;
   MemorySnapshotWin stack_;
   ProcessReaderWin::Thread thread_;