Small fix in chrome://history: mark checkbox as HTMLInputElement

R=dbeam@chromium.org
BUG=393873
TEST=GYP_GENERATORS=ninja gyp --depth . third_party/closure_compiler/compiled_resources.gyp && ninja -C out/Default

Review URL: https://codereview.chromium.org/545303003

Cr-Commit-Position: refs/heads/master@{#293627}

chrome/browser/resources/history/history.js

@@ -2180,7 +2180,8 @@ function entryBoxClick(event) {
         return;
     }
   }
-  var checkbox = $(event.currentTarget.getAttribute('for'));
+  var checkbox = assertInstanceof($(event.currentTarget.getAttribute('for')),
+                                  HTMLInputElement);
   checkbox.checked = !checkbox.checked;
   handleCheckboxStateChange(checkbox, event.shiftKey);
   // We don't want to focus on the checkbox.