DumpAccessibilityTreeTest.AccessibilitySource is failing.

Skipping the test case for source tag. This might be failing because still there are issues related with audio tags control attribute.

BUG=411987

Review URL: https://codereview.chromium.org/585093004

Cr-Commit-Position: refs/heads/master@{#295930}

content/test/data/accessibility/source-expected-android.txt

+#<skip -- crbug.com/281952>
 android.webkit.WebView focusable focused scrollable
     android.view.View
         android.view.View focusable name='0:00'

content/test/data/accessibility/source-expected-mac.txt

+#<skip -- crbug.com/281952>
 AXWebArea
     AXGroup
         AXUnknown AXTitle='0:00'

content/test/data/accessibility/source-expected-win.txt

+#<skip -- crbug.com/281952>
 ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
     IA2_ROLE_SECTION READONLY
         ROLE_SYSTEM_CLIENT name='0:00' FOCUSABLE