Disable EnterPictureInPictureThenFullscreen test on ChromeOS.

This was supposed to have been disabled on ChromeOS in
https://chromium-review.googlesource.com/c/chromium/src/+/1315207
but the CL disabled it on all platforms *except* for ChromeOS.
Removing the "!" from "!defined(OS_CHROMEOS)" because this has kept
flaking, e.g. https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/8738

TBR=mlamouri@chromium.org, vitaliii@chromium.org
NOTRY=True

Bug: 901235
Change-Id: I78ece1196efee61aa9c5fd66fd166290e3d8fbd6
Reviewed-on: https://chromium-review.googlesource.com/c/1319713
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605663}

chrome/browser/picture_in_picture/picture_in_picture_window_controller_browsertest.cc

@@ -1138,7 +1138,7 @@ IN_PROC_BROWSER_TEST_F(PictureInPictureWindowControllerBrowserTest,
 }
 
 // Flaky, crbug.com/901235.
-#if !defined(OS_CHROMEOS)
+#if defined(OS_CHROMEOS)
 #define MAYBE_EnterPictureInPictureThenFullscreen \
   DISABLED_EnterPictureInPictureThenFullscreen
 #else