Initialize STACKFRAME64 for ARM64

Bug: chromium:972228,893460

Change-Id: I0c77d19e70c337d70bc55f65f6893d5bbe18407b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1648670
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

Author: ThomsonTan

util/windows/third_party/StackWalker/src/StackWalker.cpp

@@ -1121,6 +1121,14 @@ BOOL StackWalker::ShowCallstack(HANDLE                    hThread,
   s.AddrBStore.Mode = AddrModeFlat;
   s.AddrStack.Offset = c.IntSp;
   s.AddrStack.Mode = AddrModeFlat;
+#elif _M_ARM64
+  imageType = IMAGE_FILE_MACHINE_ARM64;
+  s.AddrPC.Offset = c.Pc;
+  s.AddrPC.Mode = AddrModeFlat;
+  s.AddrFrame.Offset = c.Fp;
+  s.AddrFrame.Mode = AddrModeFlat;
+  s.AddrStack.Offset = c.Sp;
+  s.AddrStack.Mode = AddrModeFlat;
 #else
 #error "Platform not supported!"
 #endif