LibWeb+LibWebView+WebContent: Implement more <input type=file> behavior

We had previous implemented some plumbing for file input elements in
commit 636602a54e.

This implements the return path for chromes to inform WebContent of the
file(s) the user selected. This patch includes a dummy implementation
for headless-browser to enable testing.

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

@@ -0,0 +1,32 @@
+<input id="input1" type="file" />
+<input id="input2" type="file" multiple />
+<script src="./include.js"></script>
+<script type="text/javascript">
+    const runTest = async id => {
+        let input = document.getElementById(id);
+
+        return new Promise(resolve => {
+            input.addEventListener("input", async () => {
+                println(`${id}:`);
+
+                for (let i = 0; i < input.files.length; ++i) {
+                    const file = input.files.item(i);
+                    const text = await file.text();
+
+                    println(`${file.name}: ${text}`);
+                }
+
+                resolve();
+            });
+
+            internals.dispatchUserActivatedEvent(input, new Event("mousedown"));
+            input.showPicker();
+        });
+    };
+
+    asyncTest(async done => {
+        await runTest("input1");
+        await runTest("input2");
+        done();
+    });
+</script>