[PaintWorklet] Build right paint callback according to paint_arguments

In my previous CL:
https://chromium-review.googlesource.com/c/chromium/src/+/879110

I simply did null check for the |paint_arguments|, and return a nullptr
when it is null. There is a better way to handle it, which is to build
the paint callback function without the |paint_arguments| if it is null.

This CL should not change any behavior. We can use the existing tests
to verify this. We already have a PaintWorkletTest for that and a bunch
of layout tests to ensure the correct behavior.

Bug: 803026, 804206
Change-Id: I07b2f58dfe88ccbb5ac27d7268eb228ea101f5fc
Reviewed-on: https://chromium-review.googlesource.com/880886Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531396}

third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.cpp

@@ -69,9 +69,6 @@ scoped_refptr<Image> CSSPaintDefinition::Paint(
     const ImageResourceObserver& client,
     const IntSize& container_size,
     const CSSStyleValueVector* paint_arguments) {
-  if (!paint_arguments)
-    return nullptr;
-
   // TODO: Break dependency on LayoutObject. Passing the Node should work.
   const LayoutObject& layout_object = static_cast<const LayoutObject&>(client);
 
@@ -104,11 +101,19 @@ scoped_refptr<Image> CSSPaintDefinition::Paint(
                                                native_invalidation_properties_,
                                                custom_invalidation_properties_);
 
-  v8::Local<v8::Value> argv[] = {
-      ToV8(rendering_context, script_state_->GetContext()->Global(), isolate),
-      ToV8(paint_size, script_state_->GetContext()->Global(), isolate),
-      ToV8(style_map, script_state_->GetContext()->Global(), isolate),
-      ToV8(*paint_arguments, script_state_->GetContext()->Global(), isolate)};
+  Vector<v8::Local<v8::Value>, 4> argv;
+  if (paint_arguments) {
+    argv = {
+        ToV8(rendering_context, script_state_->GetContext()->Global(), isolate),
+        ToV8(paint_size, script_state_->GetContext()->Global(), isolate),
+        ToV8(style_map, script_state_->GetContext()->Global(), isolate),
+        ToV8(*paint_arguments, script_state_->GetContext()->Global(), isolate)};
+  } else {
+    argv = {
+        ToV8(rendering_context, script_state_->GetContext()->Global(), isolate),
+        ToV8(paint_size, script_state_->GetContext()->Global(), isolate),
+        ToV8(style_map, script_state_->GetContext()->Global(), isolate)};
+  }
 
   v8::Local<v8::Function> paint = paint_.NewLocal(isolate);
 
@@ -117,7 +122,7 @@ scoped_refptr<Image> CSSPaintDefinition::Paint(
 
   V8ScriptRunner::CallFunction(paint,
                                ExecutionContext::From(script_state_.get()),
-                               instance, WTF_ARRAY_LENGTH(argv), argv, isolate);
+                               instance, argv.size(), argv.data(), isolate);
 
   // The paint function may have produced an error, in which case produce an
   // invalid image.

third_party/WebKit/Source/modules/csspaint/PaintWorkletTest.cpp

@@ -141,8 +141,8 @@ TEST_F(PaintWorkletTest, GarbageCollectionOfCSSPaintDefinition) {
 // This is a crash test for crbug.com/803026. At some point, we shipped the
 // CSSPaintAPI without shipping the CSSPaintAPIArguments, the result of it is
 // that the |paint_arguments| in the CSSPaintDefinition::Paint() becomes
-// nullptr and we need to null check that. This is a regression test to ensure
-// that we don't crash.
+// nullptr and the renderer crashes. This is a regression test to ensure that
+// we will never crash.
 TEST_F(PaintWorkletTest, PaintWithNullPaintArguments) {
   PaintWorkletGlobalScope* global_scope = GetProxy()->global_scope();
   global_scope->ScriptController()->Evaluate(
@@ -157,7 +157,7 @@ TEST_F(PaintWorkletTest, PaintWithNullPaintArguments) {
   const IntSize container_size(100, 100);
   scoped_refptr<Image> image =
       definition->Paint(*observer, container_size, nullptr);
-  EXPECT_EQ(image, nullptr);
+  EXPECT_NE(image, nullptr);
 }
 
 // In this test, we set a list of "paints_to_switch" numbers, and in each frame,