Disable take_screenshot/iframe.py>>test_source_origin[same_origin]

...which is flaking on linux-blink-rel.

TBR: foolip@chromium.org
Bug: 1000734
Change-Id: Id76d6a44461a0d3c542162e5fb3e8e5cb9ed8e21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1794804Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695096}

third_party/blink/web_tests/WebDriverExpectations

@@ -178,3 +178,6 @@ crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/perform_actions/key_spec
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/new_session/default_values.py>>test_desired [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/take_screenshot/user_prompts.py>>test_dismiss_and_notify[capabilities0-alert] [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/take_element_screenshot/user_prompts.py>>test_ignore[capabilities0-alert] [ Failure ]
+
+# Sheriff 2019-09-10
+crbug.com/1000734 [ Linux ] external/wpt/webdriver/tests/take_screenshot/iframe.py>>test_source_origin[same_origin] [ Failure Pass ]