Fix the bug that generating searchable URL for <form> may change DOM.

Use local variable to store value of DOM element to avoid modifying DOM, which
may cause potential danger to event-based frontend frameworks.

Bug: 433824
Change-Id: Ie7d3a9d67483738e66730a794ba798c81d515e4e
Reviewed-on: https://chromium-review.googlesource.com/c/1303729Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603538}

ios/chrome/browser/search_engines/resources/search_engine.js

@@ -217,16 +217,17 @@ function generateSearchableUrl_(form) {
     if (isSubmitElement_(element)) {
       // Only append the active submit element's name-value pair.
       if (element === activeSubmitElement) {
+        var value = element.value;
         // <input type="submit"> will have "Submit" as default "value" when
         // submitted with empty "value" and non-empty "name". This probably
         // comes from the default label text of <input type="submit">:
         //   https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/submit
-        if (element.tagName == 'INPUT' && !element.value) {
-          element.value = 'Submit';
+        if (element.tagName == 'INPUT' && !value) {
+          value = 'Submit';
         }
         queryArgs.push(
             encodeFormData_(element.name) + '=' +
-            encodeFormData_(element.value));
+            encodeFormData_(value));
       }
       continue;
     }