Use Chai for assertions in Reader Mode's JS tests.

Bug: 1027612
Change-Id: Id22e86cb78ba35af83a46a27b46a01227fce2c0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163328Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Meredith Lane <meredithl@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#766498}

components/BUILD.gn

@@ -562,6 +562,7 @@ if (!is_ios && !is_fuchsia) {
       "dom_distiller/core/javascript/",
       "security_state/content/testdata/",
       "//content/test/data/",
+      "//third_party/chaijs",
       "//third_party/dom_distiller_js/dist/test/data/",
       "//third_party/mocha",
     ]

components/dom_distiller/content/browser/test/test_util.cc

@@ -44,6 +44,7 @@ void SetUpTestServerWithoutStarting(EmbeddedTestServer* server) {
       root_dir.AppendASCII("components/dom_distiller/core/javascript"));
   server->ServeFilesFromDirectory(
       root_dir.AppendASCII("components/test/data/dom_distiller"));
+  server->ServeFilesFromDirectory(root_dir.AppendASCII("third_party/chaijs"));
   server->ServeFilesFromDirectory(root_dir.AppendASCII("third_party/mocha"));
 }
 
@@ -60,6 +61,9 @@ FakeDistilledPage::FakeDistilledPage(EmbeddedTestServer* server)
   // DomDistillerRequestViewBase::SendCommonJavaScript(); however, this method
   // is impractical to use in testing.
   AppendScriptFile("dom_distiller_viewer.js");
+
+  // Also load test helper scripts.
+  AppendScriptFile("chai.js");
   AppendScriptFile("mocha.js");
   AppendScriptFile("test_util.js");
 }

components/test/data/dom_distiller/test_util.js

@@ -12,6 +12,7 @@ function TestReporter(runner) {
     passes++;
   });
 
+  // TODO(crbug.com/1027612): Show diff between actual and expected results.
   runner.on('fail', function(test, err) {
     failures++;
     let message = 'Mocha test failed: ' + test.fullTitle() + '\n';