Mark pointerevent_pointermove_on_chorded_mouse_button_when_locked as flaky on Linux

TBR=lanwei@chromium.org

Bug: 1025944
Change-Id: I059356aa818dce87429db6120e005515d747a3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260852Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781359}

third_party/blink/web_tests/TestExpectations

@@ -6870,3 +6870,4 @@ crbug.com/1098067 [ Fuchsia ] virtual/text-antialias/bidi-isolate-nextlinebreak-
 
 # Sheriff 2020-06-23
 crbug.com/1098136 [ Linux ] virtual/android/fullscreen/video-fixed-background.html [ Failure ]
+crbug.com/1025944 [ Linux ] external/wpt/pointerevents/pointerlock/pointerevent_pointermove_on_chorded_mouse_button_when_locked.html [ Pass Failure ]
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