[gn] Allow building a snapshot with unwinding information.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index e13162ad4ac765cef1083b8c50226274bd3bda25..c744850f81ef07b7024be061b53fa86a0c3cfb4a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -79,6 +79,10 @@ declare_args() {
   # Sets -dV8_CONCURRENT_MARKING
   v8_enable_concurrent_marking = false
 
+  # Build the snapshot with unwinding information for perf.
+  # Sets -dV8_USE_SNAPSHOT_WITH_UNWINDING_INFO.
+  v8_perf_prof_unwinding_info = false
+
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
   # tools/gen-postmortem-metadata.py for details.
@@ -256,6 +260,9 @@ config("features") {
   }
   if (v8_use_snapshot) {
     defines += [ "V8_USE_SNAPSHOT" ]
+    if (v8_perf_prof_unwinding_info) {
+      defines += [ "V8_USE_SNAPSHOT_WITH_UNWINDING_INFO" ]
+    }
   }
   if (v8_use_external_startup_data) {
     defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
@@ -743,6 +750,10 @@ action("run_mksnapshot") {
     ]
   }
 
+  if (v8_perf_prof_unwinding_info) {
+    args += [ "--perf-prof-unwinding-info" ]
+  }
+
   if (v8_use_external_startup_data) {
     outputs += [ "$root_out_dir/snapshot_blob.bin" ]
     args += [