[Chromedriver] Disable prompt dialog tests on HEAD.

Bug: chromedriver:1910
Change-Id: I66d1fdaab5c1b87deffc3c894d9cdad69c9a39a9
Reviewed-on: https://chromium-review.googlesource.com/599112Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Gauri Manikpure <gmanikpure@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491862}

chrome/test/chromedriver/test/test_expectations

@@ -137,6 +137,10 @@ _REVISION_NEGATIVE_FILTER['HEAD'] = [
     'CombinedInputActionsTest.testMouseMovementWorksWhenNavigatingToAnotherPage',
     # https://bugs.chromium.org/p/chromedriver/issues/detail?id=1138
     'AlertsTest.testSettingTheValueOfAnAlertThrows',
+    # https://bugs.chromium.org/p/chromedriver/issues/detail?id=1910
+    'AlertsTest.testPromptShouldUseDefaultValueIfNoKeysSent',
+    'UnexpectedAlertBehaviorTest.canAcceptUnhandledAlert',
+    'UnexpectedAlertBehaviorTest.canSpecifyUnhandledAlertBehaviourUsingCapabilities',
 ]
 
 _OS_NEGATIVE_FILTER = {}