Mark upcoming web animations WPT test as flaky.

external/wpt/web-animations/timing-model/timelines/document-timelines.html
is currently failing on the {linux,mac,win}_chromium_rel_ng bots even though
it passes the Blink trybots. Do this to unblock WPT imports.

TBR=ericwilligers@chromium.org

Bug: 785940
Change-Id: Ia777fcae000aa563cdd9182c21680c5c83913f57
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/774799Reviewed-by: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#517080}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2533,6 +2533,8 @@ crbug.com/666703 external/wpt/html/browsers/sandboxing/sandbox-disallow-same-ori
 crbug.com/626703 external/wpt/html/dom/documents/dom-tree-accessors/Document.currentScript.html [ Timeout ]
 crbug.com/626703 external/wpt/html/browsers/history/the-location-interface/location-protocol-setter-non-broken.html [ Failure Pass ]
 
+crbug.com/785940 external/wpt/web-animations/timing-model/timelines/document-timelines.html [ Failure Pass ]
+
 crbug.com/781595 external/wpt/html/dom/documents/resource-metadata-management/document-lastModified-01.html [ Failure ]
 
 crbug.com/655458 external/wpt/workers/constructors/SharedWorker/undefined-arguments.html [ Failure ]