Update layout test expectations when using libc++

third_party/WebKit/LayoutTests/TestExpectations

Index: third_party/WebKit/LayoutTests/TestExpectations
diff --git a/third_party/WebKit/LayoutTests/TestExpectations b/third_party/WebKit/LayoutTests/TestExpectations
index 1b9b127ddc4d8e229358996a734e3c016f64c02c..bec394e11e86f2f30d9753e6d2a73aa282375438 100644
--- a/third_party/WebKit/LayoutTests/TestExpectations
+++ b/third_party/WebKit/LayoutTests/TestExpectations
@@ -2992,6 +2992,9 @@ crbug.com/725455 [ Linux ] nfc/watch.html [ Failure Pass Timeout ]
 # Sheriff failures 2017-05-29
 crbug.com/727252 [ Win7 ] external/wpt/media-source/mediasource-endofstream.html [ Pass Timeout ]
 
+# Failures when using libc++.  Rebaseline after landing https://codereview.chromium.org/2933573002/
+crbug.com/734873 [ Linux ] fast/backgrounds/size/contain-and-cover.html [ Pass Failure ]
+
 # Compositor worker tests are flaky, crashing and timing out
 crbug.com/727505 [ Linux Mac Win ] virtual/threaded/fast/compositorworker/basic-plumbing-main-to-worker.html [ Pass Failure Timeout Crash ]
 crbug.com/727505 [ Linux Mac Win ] virtual/threaded/fast/compositorworker/basic-plumbing-worker-to-main.html [ Pass Failure Timeout Crash ]