LayoutTests: drop obsolete NeedsManualRebaseline

BigInts have landed and can be tested regularly.

Bug: v8:6791
Change-Id: Ida4a9648fe1b49a7a905f86d6925515c0c78ed80
Reviewed-on: https://chromium-review.googlesource.com/1038406
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555194}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4540,9 +4540,6 @@ crbug.com/816914 [ Mac ] fast/canvas/canvas-drawImage-live-video.html [ Failure
 crbug.com/816766 [ Mac ] fast/dom/Window/window-focus-self.html [ Failure Pass ]
 crbug.com/817167 http/tests/devtools/oopif/oopif-cookies-refresh.js [ Failure Timeout Pass ]
 
-# Simply remove this line when BigInts have landed in Chromium; ...-expected.txt is already correct.
-crbug.com/v8/6791 external/wpt/html/infrastructure/safe-passing-of-structured-data/structured_clone_bigint.html [ NeedsManualRebaseline ]
-
 # Sheriff 2018-03-01
 crbug.com/817678 [ Win ] http/tests/devtools/console/console-group-similar.js [ Failure Pass ]