Disable two flakey screenshot tests.

Disabling contrib.cluster_telemetry.screenshot_unittest.ScreenshotUnitTest.
testScreenshot and external/wpt/webdriver/tests/take_element_screenshot/iframe.py>>test_source_origin to de-jank the linux-trusty-rel bot.

TBR=keishi@chromium.org

Bug: 1023255
Change-Id: Ie8fbb182ba5d2d3c4df5282db62b72b622a26b86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1908715
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714278}

third_party/blink/web_tests/WebDriverExpectations

@@ -77,6 +77,7 @@ crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/interface.html [ Failure
 crbug.com/626703 [ Mac ] external/wpt/webdriver/tests/interface.html [ Failure ]
 crbug.com/626703 [ Win ] external/wpt/webdriver/tests/interface.html [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/take_element_screenshot/iframe.py>>test_source_origin[cross_origin] [ Failure ]
+crbug.com/1023255 [ Linux ] external/wpt/webdriver/tests/take_element_screenshot/iframe.py>>test_source_origin[same_origin] [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/take_screenshot/iframe.py>>test_source_origin[cross_origin] [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/switch_to_frame/cross_origin.py>>test_nested_cross_origin_iframe [ Failure ]
 crbug.com/626703 [ Linux ] external/wpt/webdriver/tests/switch_to_frame/cross_origin.py>>test_cross_origin_iframe [ Failure ]

tools/perf/contrib/cluster_telemetry/screenshot_unittest.py

@@ -11,7 +11,7 @@ from contrib.cluster_telemetry import screenshot
 
 
 class ScreenshotUnitTest(legacy_page_test_case.LegacyPageTestCase):
-  @decorators.Enabled('linux')
+  @decorators.Disabled('linux')
   def testScreenshot(self):
     # Screenshots for Cluster Telemetry purposes currently only supported on
     # Linux platform.