Fix flaky EmailAliasesBrowserTest.ContextMenuAuthorized (#36161)

The test intermittently failed with "deepQuery is not defined" because
InjectHelpers() could execute on the bubble's WebContents before its
WebUI page (chrome://email-aliases.panel/) had finished loading. When
the WebUI document committed after injection, the JavaScript context
was replaced and deepQuery was lost.

Add WaitForLoadStop() in InjectHelpers() to ensure the target
WebContents has finished loading before injecting helper functions.
This matches the pattern used by brave_wallet_tab_helper_browsertest.

Resolves brave/brave-browser#54889

browser/email_aliases/email_aliases_browsertest.cc

@@ -178,6 +178,7 @@ class EmailAliasesBrowserTestBase : public InProcessBrowserTest {
   }
 
   void InjectHelpers(content::WebContents* contents) {
+    ASSERT_TRUE(content::WaitForLoadStop(contents));
     constexpr char kDeepQuery[] = R"js(
       function deepQuery(selector) {
         const query = (root, selector) =>{