[Extensions] Maybe de-flake ExtensionBindingsApiTest.LastError

Attempt to deflake ExtensionBindingsApiTest.LastError. The test was
failing with a segmentation fault, which most likely happened when
trying to de-reference an ExtensionHost that could be null. Try to
fix this by:
- ASSERTing the ExtensionHost exists (at least giving us a good error
  message if it does not)
- Waiting for a "ready" message from the extension, which guarantees
  the background page has been created.

Bug: 1035011

Change-Id: I13510ea649f6488f216bf50bd4cf00bb9d521292
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1972263Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726000}

chrome/browser/extensions/extension_bindings_apitest.cc

@@ -87,15 +87,17 @@ IN_PROC_BROWSER_TEST_F(ExtensionBindingsApiTest,
 
 // Tests that an error raised during an async function still fires
 // the callback, but sets chrome.runtime.lastError.
-// Flaky on all platforms: https://crbug.com/1035011
-IN_PROC_BROWSER_TEST_F(ExtensionBindingsApiTest, DISABLED_LastError) {
+IN_PROC_BROWSER_TEST_F(ExtensionBindingsApiTest, LastError) {
+  ExtensionTestMessageListener ready_listener("ready", /*will_reply=*/false);
   ASSERT_TRUE(LoadExtension(
       test_data_dir_.AppendASCII("bindings").AppendASCII("last_error")));
+  ASSERT_TRUE(ready_listener.WaitUntilSatisfied());
 
   // Get the ExtensionHost that is hosting our background page.
   extensions::ProcessManager* manager =
       extensions::ProcessManager::Get(browser()->profile());
   extensions::ExtensionHost* host = FindHostWithPath(manager, "/bg.html", 1);
+  ASSERT_TRUE(host);
 
   bool result = false;
   ASSERT_TRUE(content::ExecuteScriptAndExtractBool(host->host_contents(),

chrome/test/data/extensions/api_test/bindings/last_error/bg.js

@@ -54,6 +54,4 @@ function finish() {
   succeed();
 }
 
-document.documentElement.addEventListener("click", function() {
-  testLastError();
-}, true);
+chrome.test.sendMessage('ready');