Fix WebViewTest.TestContextMenu on official builders.

We stopped showing "Inspect Element" on dev/beta/stable, therefore
TestContextMenu does not seem to find any menu item when right-click
is performed.
This CL makes sure we always have an <input> on the right-click target
so that we have some items to show in the context menu.

BUG=470300
Test=None, internal test only change.

Review URL: https://codereview.chromium.org/1016033010

Cr-Commit-Position: refs/heads/master@{#322326}

chrome/test/data/extensions/platform_apps/web_view/context_menus/basic/guest.js

@@ -25,4 +25,22 @@ var onPostMessageReceived = function(e) {
     return;
   }
 };
+
+var setup = function() {
+  // Make sure there is always stuff to show in context menu by
+  // inserting an <input> element.
+  // Note that we don't always show "Inspect element", so this is
+  // necessary.
+  var div = document.createElement('div');
+  div.style.position = 'absolute';
+  div.style.top = 0;
+  div.style.left = 0;
+  var input = document.createElement('input');
+  div.appendChild(input);
+  document.body.style.padding = 0;
+  document.body.style.margin = 0;
+  document.body.appendChild(div);
+};
+
+setup();
 window.addEventListener('message', onPostMessageReceived, false);