ambient: clean up test helper class.

Removed the |UpdateDisplay()| call during test set up, since the
effected test cases have been disabled due to flakiness.

Bug: b/158617675
Test: manully.
Change-Id: I4ff9ec8fd0474bbb85d19e658d5a3aa44e6f435f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250700Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779538}

ash/ambient/test/ambient_ash_test_base.cc

@@ -26,7 +26,6 @@ void AmbientAshTestBase::SetUp() {
   ambient_client_ = std::make_unique<TestAmbientClient>();
 
   AshTestBase::SetUp();
-  UpdateDisplay("200x100");
 
   // Need to reset first and then assign the TestPhotoClient because can only
   // have one instance of AmbientBackendController.