Testexpectations: Add back a line for a flaky test

This line was commented out in http://crrev.com/122463005
And removed (incorrectly) by auto-rebaseline bot in:
http://crrev.com/134313003

BUG=331619
[email protected]

Review URL: https://codereview.chromium.org/137403003

git-svn-id: svn://svn.chromium.org/blink/trunk@165010 bbb929c8-8fbe-4397-9dbb-9b2b20218538
diff --git a/third_party/WebKit/LayoutTests/TestExpectations b/third_party/WebKit/LayoutTests/TestExpectations
index 372363a65..d1981c3 100644
--- a/third_party/WebKit/LayoutTests/TestExpectations
+++ b/third_party/WebKit/LayoutTests/TestExpectations
@@ -737,6 +737,8 @@
 crbug.com/312925 inspector-protocol/heap-profiler/heap-snapshot-with-detached-dom-tree.html [ Failure Pass ]
 crbug.com/313783 [ Debug ] fast/css/getComputedStyle/getComputedStyle-resolved-values.html [ Crash Pass ]
 
+crbug.com/313846 [ Mac Win ] http/tests/images/webp-partial-load.html [ ImageOnlyFailure Timeout Pass ]
+
 crbug.com/316150 inspector/timeline/timeline-network-received-data.html [ Failure Pass ]
 
 crbug.com/315514 [ Linux ] fast/workers/storage/test-authorizer-sync.html [ Failure ]