WebUI: Prevent "not null" cast for querySelectorAll() calls, part 2.

A "!" not-null operator after querySelectorAll() is unnecessary since,
unlike querySelector(), it always returns a non-null value.

In this part adding an ESLint check to automatically catch such
violations.

Bug: 720034
Change-Id: I0460549c0038d6cf56d2aeb9db13ebf5ea7d3447
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5250843
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1254271}

.eslintrc.js

@@ -76,6 +76,11 @@ module.exports = {
       // https://google.github.io/styleguide/tsguide.html#return-type-only-generics
       'selector': 'TSAsExpression > CallExpression > MemberExpression[property.name=/^querySelectorAll$/]',
       'message': 'Don\'t use \'querySelectorAll(...) as Type\'. Use the type parameter, \'querySelectorAll<Type>(...)\' instead',
+    },
+    {
+      // Prevent a common misuse of "!" operator.
+      "selector": "TSNonNullExpression > CallExpression > MemberExpression[property.name=/^querySelectorAll$/]",
+      "message": "Remove unnecessary \"!\" non-null operator after querySelectorAll(). It always returns a non-null result",
     }],
     'no-throw-literal': 'error',
     'no-trailing-spaces': 'error',