Fix crash in shell.cc when reporting exceptions that aren't Errors

BUG=chromium:534746
R=vogelheim@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1410333004

Cr-Commit-Position: refs/heads/master@{#31414}

samples/shell.cc

@@ -412,9 +412,11 @@ void ReportException(v8::Isolate* isolate, v8::TryCatch* try_catch) {
       fprintf(stderr, "^");
     }
     fprintf(stderr, "\n");
-    v8::String::Utf8Value stack_trace(
-        try_catch->StackTrace(context).ToLocalChecked());
-    if (stack_trace.length() > 0) {
+    v8::Local<v8::Value> stack_trace_string;
+    if (try_catch->StackTrace(context).ToLocal(&stack_trace_string) &&
+        stack_trace_string->IsString() &&
+        v8::Local<v8::String>::Cast(stack_trace_string)->Length() > 0) {
+      v8::String::Utf8Value stack_trace(stack_trace_string);
       const char* stack_trace_string = ToCString(stack_trace);
       fprintf(stderr, "%s\n", stack_trace_string);
     }