LibWeb: Reverse check for whether a FilesList index is supported

This fixes for..of iteration of a FilesList object.

Tests/LibWeb/Text/expected/input-file.txt

@@ -1,7 +1,12 @@
 Select file...file1 Select files...4 files selected.   input1:
-file1: Contents for file1
+file1 (index iteration): Contents for file1
+file1 (for..of iteration): Contents for file1
 input2:
-file1: Contents for file1
+file1 (index iteration): Contents for file1
-file2: Contents for file2
+file2 (index iteration): Contents for file2
-file3: Contents for file3
+file3 (index iteration): Contents for file3
-file4: Contents for file4
+file4 (index iteration): Contents for file4
+file1 (for..of iteration): Contents for file1
+file2 (for..of iteration): Contents for file2
+file3 (for..of iteration): Contents for file3
+file4 (for..of iteration): Contents for file4

Tests/LibWeb/Text/input/input-file.html

@@ -13,7 +13,13 @@
                     const file = input.files.item(i);
                     const text = await file.text();
 
-                    println(`${file.name}: ${text}`);
+                    println(`${file.name} (index iteration): ${text}`);
+                }
+
+                for (let file of input.files) {
+                    const text = await file.text();
+
+                    println(`${file.name} (for..of iteration): ${text}`);
                 }
 
                 resolve();

Userland/Libraries/LibWeb/FileAPI/FileList.cpp

@@ -41,7 +41,7 @@ bool FileList::is_supported_property_index(u32 index) const
     if (m_files.is_empty())
         return false;
 
-    return m_files.size() < index;
+    return index < m_files.size();
 }
 
 WebIDL::ExceptionOr<JS::Value> FileList::item_value(size_t index) const