PostMessageTest: retire use of zero-argument port.postMessage().

In testPostUnsupportedWebMessageToApp, avoid invoking postMessage()
without any arguments. At least one is required for
MessagePort.postMessage().

R=sgurun@chromium.org
BUG=339441

Review URL: https://codereview.chromium.org/1003603004

Cr-Commit-Position: refs/heads/master@{#321855}

android_webview/javatests/src/org/chromium/android_webview/test/PostMessageTest.java

@@ -1031,7 +1031,6 @@ public class PostMessageTest extends AwTestBase {
             + "    <script>"
             + "        onmessage = function (e) {"
             + "            e.source.postMessage('" + HELLO + "', '*');"
-            + "            e.ports[0].postMessage();"
             + "            e.ports[0].postMessage(null);"
             + "            e.ports[0].postMessage(undefined);"
             + "            e.ports[0].postMessage(NaN);"