Disable two tast ui tests due to lack of private variable access.

Both ui.ExistingUserLogin and ui.SigninProfileExtension
were recently promoted to critical tests, but failed since
they need to access some variables in tast-tests-private,
which are not available in Chromium CQ.

Bug: 1148036
Change-Id: I1c3e4e4a53555aa769a8e3e17a03288cf6b2b46a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533234Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826615}

chromeos/BUILD.gn

@@ -321,6 +321,10 @@ if (is_chromeos_device) {
       "inputs.VirtualKeyboardDeadKeys.french_stable",
       "inputs.VirtualKeyboardTypingApps.stable",
       "inputs.VirtualKeyboardTypingBrowser.stable",
+
+      # crbug.com/1148036
+      "ui.ExistingUserLogin",
+      "ui.SigninProfileExtension",
     ]
   }