Disable two webdriver tests

Disable two tests which consistently fail.

- external/wpt/webdriver/tests/take_screenshot/iframe.py>>test_frame_content
- external/wpt/webdriver/tests/take_element_screenshot/iframe.py>>test_frame_element

TBR=johnchen@chromium.org,jbudorick@chromium.org,robliao@chromium.org

Bug: 1039578
Change-Id: Ie7ea1967a1e70b1abe3199e899c2cd26b93233ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989292Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728834}

third_party/blink/web_tests/TestExpectations

@@ -7172,3 +7172,7 @@ crbug.com/963183 http/tests/devtools/sources/debugger-ui/script-formatter-breakp
 crbug.com/963183 http/tests/devtools/sources/debugger/js-with-inline-stylesheets.js [ Pass Failure ]
 crbug.com/963183 http/tests/devtools/startup/sources/debugger/script-formatter-breakpoints-1.js [ Pass Failure ]
 crbug.com/963183 http/tests/devtools/startup/sources/debugger/script-formatter-breakpoints-inline-with-sourceURL.js [ Pass Failure ]
+
+# Sheriff 2020-01-06
+crbug.com/1039578 external/wpt/webdriver/tests/take_screenshot/iframe.py>>test_frame_content [ Pass Failure ]
+crbug.com/1039578 external/wpt/webdriver/tests/take_element_screenshot/iframe.py>>test_frame_element [ Pass Failure ]