Disable TabletModeTransitionTest.EnterExit

I enabled this test since it passes all of 20 runs locally, but
it turns out it is still flaky on our dbg buildbots. I guess
that's because of the performance regression noted as
crbug.com/1054489 and it's passing since my workstation performs
well. Anyways, let's disable it for now.

Bug: 1054489, 1057868
Test: none
Change-Id: Iea764beb4000bd2dcd8361dad011404a92995db5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111057Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751823}

chrome/browser/ui/ash/tablet_mode_transition_interactive_uitest.cc

@@ -61,7 +61,9 @@ class TabletModeTransitionTest : public UIPerformanceTest {
   DISALLOW_COPY_AND_ASSIGN(TabletModeTransitionTest);
 };
 
-IN_PROC_BROWSER_TEST_F(TabletModeTransitionTest, EnterExit) {
+// Flaky possibly due to https://crbug.com/1054489
+// TODO(sammiequon, mukai): re-enable this. See also https://crbug.com/1057868
+IN_PROC_BROWSER_TEST_F(TabletModeTransitionTest, DISABLED_EnterExit) {
   // Activate the first window. The top window is the only window which animates
   // and is the one we should check to see if the tablet animation has finished.
   Browser* browser = BrowserList::GetInstance()->GetLastActive();