Re-enable two WPT screenshot tests

These failures might be due to crrev.com/692284, which has been
reverted. Re-enabling the tests.

Bug: 1000734
Change-Id: I8d4c7bfdce6be7f64ee08f83de08a134a45a41e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846815Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703734}

third_party/blink/web_tests/WebDriverExpectations

@@ -169,8 +169,3 @@ 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 ]
-# Sheriff 2019-09-11
-crbug.com/1000734 [ Linux ] external/wpt/webdriver/tests/take_screenshot/screenshot.py>>test_format_and_dimensions [ Failure Pass ]