Disable a test on single process mash content browser tests

Disabling due to flake:
  NavigationControllerBrowserTest.UtilizationOfSpareRenderProcessHost

TBR=xiyuan@chromium.org

Bug: 888156
Change-Id: Ice1001691d141c2ffb2cec2399dffe8855acdf3a
Reviewed-on: https://chromium-review.googlesource.com/1246145Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594381}

testing/buildbot/filters/chromeos.single_process_mash.content_browsertests.filter

@@ -121,3 +121,7 @@
 
 # The test times out. https://crbug.com/884393
 -WebRtcCaptureFromElementBrowserTest.VerifyCanvasCaptureOffscreenCanvasFrames
+
+
+# https://crbug.com/888156
+-NavigationControllerBrowserTest.UtilizationOfSpareRenderProcessHost