[WebLayer] Allow GoogleAccountsCallback to be called in incognito

This allows a client to block signin in incognito.

Bug: 1138458
Change-Id: I724a4dace1cdb6fc1c38f0640c344a12d4482021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470467
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817087}

weblayer/browser/google_accounts_browsertest.cc

@@ -219,11 +219,13 @@ class IncognitoGoogleAccountsBrowserTest : public GoogleAccountsBrowserTest {
 };
 
 IN_PROC_BROWSER_TEST_F(IncognitoGoogleAccountsBrowserTest,
-                       HeaderNotAddedForIncognitoBrowser) {
+                       HeaderAddedForIncognitoBrowser) {
   const std::string path =
       base::StrCat({"/echoheader?", signin::kChromeConnectedHeader});
   NavigateAndWaitForCompletion(GetGaiaURL(path), shell());
-  EXPECT_EQ(GetBody(), "None");
+  EXPECT_EQ(GetBody(),
+            "source=WebLayer,mode=3,enable_account_consistency=true,"
+            "consistency_enabled_by_default=false");
 }
 
 }  // namespace weblayer

weblayer/browser/signin_url_loader_throttle.cc

@@ -60,7 +60,7 @@ SigninURLLoaderThrottle::~SigninURLLoaderThrottle() = default;
 std::unique_ptr<SigninURLLoaderThrottle> SigninURLLoaderThrottle::Create(
     content::BrowserContext* browser_context,
     content::WebContents::Getter web_contents_getter) {
-  if (browser_context->IsOffTheRecord() || !GetDelegate(web_contents_getter))
+  if (!GetDelegate(web_contents_getter))
     return nullptr;
 
   // Use base::WrapUnique + new because of the constructor is private.