Reland "[signin] Add reauth browser tests with a fake Gaia page"

This reverts commit 2d660071.

Reason for revert: fixed the tests by increasing the dialog
interaction timeouts

Original change's description:
> Revert "[signin] Add reauth browser tests with a fake Gaia page"
> 
> This reverts commit 62ff616d.
> 
> Reason for revert: 
> Both of these are failing on MSAN:
> SigninReauthViewControllerBrowserTest.CancelReauthDialog
> SigninReauthViewControllerBrowserTest.ConfirmReauthDialog
> 
> Original change's description:
> > [signin] Add reauth browser tests with a fake Gaia page
> > 
> > This CL adds a new set of tests for the reauth dialog. These tests
> > check interactions with a Gaia Reauth endpoint that is imitated through
> > an EmbeddedTestServer.
> > 
> > The main "embedded/xreauth/chrome" endpoint operates in two modes:
> > - In the default mode, the endpoint redirects to the "challenge" URL
> > whose response can be controlled within the test by a
> > ControllableHttpResponse.
> > - If the "?done" query is present, the endpoint returns the
> > HTTP_NO_CONTENT response that signifies a successful reauth.
> > 
> > The CL also extracts common test code into test suite functions to
> > reduce the code duplication and make the tests code more readable.
> > 
> > Bug: 1045515
> > Change-Id: I89d4aac57ef25168def1341fb63b1a7498dcfa0f
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225396
> > Commit-Queue: Alex Ilin <alexilin@chromium.org>
> > Reviewed-by: David Roger <droger@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#774205}
> 
> TBR=droger@chromium.org,alexilin@chromium.org
> 
> Change-Id: I9e322b00bb98f9394b81fda2a0e62bf5db5cd9e9
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1045515, 1090451
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225852
> Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
> Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#774265}

TBR=dmazzoni@chromium.org,droger@chromium.org,alexilin@chromium.org

# Not skipping CQ checks because this is a reland.

Bug: 1045515, 1090451
Change-Id: I733694fb5790290fe6ae1e661708ddce4a7988c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228540Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774764}

chrome/browser/signin/signin_features.cc

@@ -8,3 +8,6 @@
 const base::Feature kDiceWebSigninInterceptionFeature{
     "DiceWebSigninInterception", base::FEATURE_DISABLED_BY_DEFAULT};
 #endif  // ENABLE_DICE_SUPPORT
+
+const base::Feature kSigninReauthPrompt = {"SigninReauthPrompt",
+                                           base::FEATURE_DISABLED_BY_DEFAULT};

chrome/browser/signin/signin_features.h

@@ -12,4 +12,6 @@
 extern const base::Feature kDiceWebSigninInterceptionFeature;
 #endif  // ENABLE_DICE_SUPPORT
 
+extern const base::Feature kSigninReauthPrompt;
+
 #endif  // CHROME_BROWSER_SIGNIN_SIGNIN_FEATURES_H_

chrome/browser/ui/signin_reauth_view_controller.cc

@@ -13,6 +13,7 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/signin/reauth_result.h"
 #include "chrome/browser/signin/reauth_tab_helper.h"
+#include "chrome/browser/signin/signin_features.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_navigator_params.h"
 #include "content/public/browser/browser_task_traits.h"
@@ -24,9 +25,6 @@
 
 namespace {
 
-const base::Feature kSigninReauthPrompt = {"SigninReauthPrompt",
-                                           base::FEATURE_DISABLED_BY_DEFAULT};
-
 class ReauthWebContentsObserver : public content::WebContentsObserver {
  public:
   ReauthWebContentsObserver(content::WebContents* web_contents,