Marking two fullscreen-related tests as flaky

TBR=mamir@chromium.org
Bug: 1123907

Change-Id: I5b50e8cf2b1947d2d94bb7fec906266b19e0ddc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386389Reviewed-by: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803493}

third_party/blink/web_tests/WebDriverExpectations

@@ -180,3 +180,5 @@ crbug.com/1065297 [ Linux ] external/wpt/webdriver/tests/perform_actions/pointer
 crbug.com/1065297 [ Linux ] external/wpt/webdriver/tests/perform_actions/pointer.py>>test_drag_and_drop_with_draggable_element[0] [ Failure ]
 crbug.com/1065297 [ Linux ] external/wpt/webdriver/tests/perform_actions/pointer.py>>test_drag_and_drop_with_draggable_element[300] [ Failure ]
 crbug.com/1065297 [ Linux ] external/wpt/webdriver/tests/perform_actions/key_shortcuts.py>>test_mod_a_mod_c_right_mod_v_pastes_text [ Failure ]
+crbug.com/1123907 [ Linux ] external/wpt/webdriver/tests/fullscreen_window/fullscreen.py>>test_fullscreen_twice_is_idempotent [ Failure Pass ]
+crbug.com/1123907 [ Linux ] external/wpt/webdriver/tests/fullscreen_window/stress.py>>test_stress [ Failure Pass ]
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