Fix flaky EULA load offline test

Creates a new test class for testing the offline fallback mechanism
used in Chrome OS EULA terms. The behaviour the embedded server used
for EULA tests is now determined during construction and cannot be
changed in the tests themselves.

Tested locally.

Bug: 1015948
Change-Id: I87c7196968234ebde2573d604beee132564df899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1926589
Commit-Queue: Renato Silva <rrsilva@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717697}

chrome/browser/chromeos/login/eula_browsertest.cc

@@ -129,9 +129,12 @@ class EulaTest : public OobeBaseTest {
     OobeScreenWaiter(EulaView::kScreenId).Wait();
   }
 
-  void set_allow_online_eula(bool allow) { allow_online_eula_ = allow; }
-
  protected:
+  // Used for customizing the response handler of the embedded server.
+  void set_force_http_unavailable(bool force_unavailable) {
+    force_http_unavailable_ = force_unavailable;
+  }
+
   content::WebContents* FindEulaContents() {
     // Tag the Eula webview in use with a unique name.
     constexpr char kUniqueEulaWebviewName[] = "unique-eula-webview-name";
@@ -228,18 +231,21 @@ class EulaTest : public OobeBaseTest {
     std::unique_ptr<BasicHttpResponse> http_response =
         std::make_unique<BasicHttpResponse>();
 
-    if (allow_online_eula_) {
+    if (force_http_unavailable_) {
+      http_response->set_code(net::HTTP_SERVICE_UNAVAILABLE);
+    } else {
       http_response->set_code(net::HTTP_OK);
       http_response->set_content_type("text/html");
       http_response->set_content(kFakeOnlineEula);
-    } else {
-      http_response->set_code(net::HTTP_SERVICE_UNAVAILABLE);
     }
 
     return std::move(http_response);
   }
 
-  bool allow_online_eula_ = true;
+  // The default behaviour for the embedded server is to service the
+  // online version properly. Offline tests may change this during construction
+  // of the class.
+  bool force_http_unavailable_ = false;
 
   // URL used for testing. Retrieved from the embedded server.
   std::string fake_eula_url_;
@@ -247,9 +253,27 @@ class EulaTest : public OobeBaseTest {
   DISALLOW_COPY_AND_ASSIGN(EulaTest);
 };
 
+// When testing the offline fallback mechanism, the requests reaching the
+// embedded server have to be handled differently.
+class EulaOfflineTest : public EulaTest {
+ public:
+  EulaOfflineTest() { set_force_http_unavailable(true); }
+
+  ~EulaOfflineTest() override = default;
+};
+
+// Tests that offline version is shown when the online version is not
+// accessible.
+IN_PROC_BROWSER_TEST_F(EulaOfflineTest, LoadOffline) {
+  ShowEulaScreen();
+
+  WaitForLocalWebviewLoad();
+  EXPECT_TRUE(test::GetWebViewContents({"oobe-eula-md", "crosEulaFrame"})
+                  .find(kOfflineEULAWarning) != std::string::npos);
+}
+
 // Tests that online version is shown when it is accessible.
 IN_PROC_BROWSER_TEST_F(EulaTest, LoadOnline) {
-  set_allow_online_eula(true);
   ShowEulaScreen();
 
   // Wait until the webview has finished loading.
@@ -267,18 +291,6 @@ IN_PROC_BROWSER_TEST_F(EulaTest, LoadOnline) {
   EXPECT_TRUE(webview_contents.find(kFakeOnlineEula) != std::string::npos);
 }
 
-// Tests that offline version is shown when the online version is not
-// accessible.
-IN_PROC_BROWSER_TEST_F(EulaTest, LoadOffline) {
-  set_allow_online_eula(false);
-  ShowEulaScreen();
-
-  WaitForLocalWebviewLoad();
-
-  EXPECT_TRUE(test::GetWebViewContents({"oobe-eula-md", "crosEulaFrame"})
-                  .find(kOfflineEULAWarning) != std::string::npos);
-}
-
 // Tests that clicking on "System security settings" button opens a dialog
 // showing the TPM password.
 IN_PROC_BROWSER_TEST_F(EulaTest, DisplaysTpmPassword) {