[tools/perf] Fix JavaScript interpolation in action_runner calls

tools/perf/page_sets/login_helpers/dropbox_login.py

Index: tools/perf/page_sets/login_helpers/dropbox_login.py
diff --git a/tools/perf/page_sets/login_helpers/dropbox_login.py b/tools/perf/page_sets/login_helpers/dropbox_login.py
index fda3a3a4493f5e276a4976fefebd7ec2ee39f676..c4f81c4fc48fe98fef1a7f796b2caa29f65ecaec 100644
--- a/tools/perf/page_sets/login_helpers/dropbox_login.py
+++ b/tools/perf/page_sets/login_helpers/dropbox_login.py
@@ -32,13 +32,13 @@ def LoginAccount(action_runner, credential,
 
   # Wait until the "Sign in" button is enabled and then click it.
   login_button_selector = '.login-form .login-button'
-  # TODO(catapult:#3028): Fix interpolation of JavaScript values.
   action_runner.WaitForJavaScriptCondition('''
       (function() {
-        var loginButton = document.querySelector("%s");
+        var loginButton = document.querySelector({{ selector }});
         if (!loginButton)
           return false;
         return !loginButton.disabled;
-      })();''' % login_button_selector)
+      })();''',
+      selector=login_button_selector)
   action_runner.ClickElement(selector=login_button_selector)
   action_runner.WaitForNavigate()