Re-enable EnterPictureInPictureThenFullscreen test on ChromeOS.

This CL is almost reverts of the following CL: crrev.com/c/1315207

This test has been disabled on Chrome OS because of flakiness: crbug.com/901235.

The flakiness is fixed by the recent change (crrev.com/c/1316986) so
that this CL re-enable it.

TBR=mlamouri@chromium.org, vitaliii@chromium.org
Bug: 901235
Test: trybots pass

Change-Id: I1cec695988728ae40bd1f6031135efaf07c760b3
Reviewed-on: https://chromium-review.googlesource.com/c/1325597Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606384}

chrome/browser/picture_in_picture/picture_in_picture_window_controller_browsertest.cc

@@ -1137,16 +1137,8 @@ IN_PROC_BROWSER_TEST_F(PictureInPictureWindowControllerBrowserTest,
   EXPECT_TRUE(second_controller->GetWindowForTesting()->IsVisible());
 }
 
-// Flaky, crbug.com/901235.
-#if defined(OS_CHROMEOS)
-#define MAYBE_EnterPictureInPictureThenFullscreen \
-  DISABLED_EnterPictureInPictureThenFullscreen
-#else
-#define MAYBE_EnterPictureInPictureThenFullscreen \
-  EnterPictureInPictureThenFullscreen
-#endif
 IN_PROC_BROWSER_TEST_F(PictureInPictureWindowControllerBrowserTest,
-                       MAYBE_EnterPictureInPictureThenFullscreen) {
+                       EnterPictureInPictureThenFullscreen) {
   LoadTabAndEnterPictureInPicture(browser());
 
   content::WebContents* active_web_contents =