remoteobjects: Ensure that null object return values map to null.

Bug: 794320
Change-Id: If5776962746a12481fd89426f5e13a3ab6b79214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095779Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748563}

content/public/android/java/src/org/chromium/content/browser/remoteobjects/RemoteObjectImpl.java

@@ -400,6 +400,8 @@ class RemoteObjectImpl implements RemoteObject {
             } else {
                 resultValue.setStringValue(javaStringToMojoString((String) result));
             }
+        } else if (result == null) {
+            resultValue.setSingletonValue(SingletonJavaScriptValue.NULL);
         } else {
             int objectId = objectIdAllocator.getObjectId(result);
             resultValue.setObjectId(objectId);

content/public/android/junit/src/org/chromium/content/browser/remoteobjects/RemoteObjectImplTest.java

@@ -750,6 +750,11 @@ public final class RemoteObjectImplTest {
             public Object getFoo() {
                 return foo;
             }
+
+            @TestJavascriptInterface
+            public Object getNull() {
+                return null;
+            }
         };
 
         when(mIdAllocator.getObjectId(foo)).thenReturn(42);
@@ -757,8 +762,10 @@ public final class RemoteObjectImplTest {
         RemoteObject remoteObject = newRemoteObjectImpl(target, TestJavascriptInterface.class);
         RemoteObject.InvokeMethodResponse response = mock(RemoteObject.InvokeMethodResponse.class);
         remoteObject.invokeMethod("getFoo", new RemoteInvocationArgument[] {}, response);
+        remoteObject.invokeMethod("getNull", new RemoteInvocationArgument[] {}, response);
 
         verify(response).call(resultIsObject(42));
+        verify(response).call(resultIsNull());
     }
 
     private RemoteInvocationResult resultHasError(final int error) {
@@ -777,6 +784,14 @@ public final class RemoteObjectImplTest {
         }));
     }
 
+    private RemoteInvocationResult resultIsNull() {
+        return and(resultIsOk(), ArgumentMatchers.argThat(result -> {
+            return result.value != null
+                    && result.value.which() == RemoteInvocationResultValue.Tag.SingletonValue
+                    && result.value.getSingletonValue() == SingletonJavaScriptValue.NULL;
+        }));
+    }
+
     private RemoteInvocationResult resultIsNumber(final double numberValue) {
         return and(resultIsOk(), ArgumentMatchers.argThat(result -> {
             return result.value != null