Don't pass a null v8 handle for an empty bitmap.

This causes v8 to crash, instead we pass no 3rd argument at all, which
it will handle gracefully as an undefined.

R=ishell@chromium.org

Bug: 1093391
Change-Id: I1fdfe64bef348062495e2d5d6db940f36f2db982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419141Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808505}

content/web_test/renderer/test_runner.cc

@@ -109,8 +109,8 @@ std::vector<v8::Local<v8::Value>> ConvertBitmapToV8(
   args.push_back(v8::Number::New(isolate, bitmap.info().width()));
   args.push_back(v8::Number::New(isolate, bitmap.info().height()));
   if (bitmap.isNull()) {
-    // Empty value for the bitmap.
-    args.emplace_back();
+    // The 3rd argument will be undefined (an empty argument is not valid and
+    // would crash).
     return args;
   }