Update the bug number for failing wpt shadow-dom tests

The entry pointed to a meta-bug of wpt shadow-dom test issues,
but the only remaining one has its own dedicated bug 392771.

Bug: 392771, 505364
Change-Id: Ie32ec84d3accbdeabaac79a8c59f9f3d560c48ec
Reviewed-on: https://chromium-review.googlesource.com/1103977Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567950}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2056,7 +2056,7 @@ crbug.com/404597 [ Mac ] fast/css3-text/css3-text-justify/text-justify-crash.htm
 crbug.com/404597 fast/forms/long-text-in-input.html [ Skip ]
 
 # Web Components related tests (Shadow DOM, Custom Elements) failures.
-crbug.com/505364 external/wpt/shadow-dom/untriaged/styles/test-003.html [ Failure ]
+crbug.com/392771 external/wpt/shadow-dom/untriaged/styles/test-003.html [ Failure ]
 
 crbug.com/552494 scrollbars/overflow-scrollbar-combinations.html [ Pass Failure ]
 crbug.com/552494 virtual/prefer_compositing_to_lcd_text/scrollbars/overflow-scrollbar-combinations.html [ Pass Failure ]