Mark variable-declaration-15.html flaky.

BUG=582836
NOTRY=true
TBR=suzyh@chromium.org

Review-Url: https://codereview.chromium.org/2838883003
Cr-Commit-Position: refs/heads/master@{#467254}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2138,6 +2138,7 @@ crbug.com/553838 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftest
 # We paint in an incorrect order when layers are present
 crbug.com/370604 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-paint-ordering-002.xhtml [ Failure ]
 
+crbug.com/582836 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/variables/variable-declaration-15.html [ Failure Pass ]
 crbug.com/582836 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/variables/variable-external-font-face-01.html [ Pass Failure ]
 crbug.com/582836 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/variables/variable-font-face-01.html [ Failure ]
 crbug.com/582836 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/variables/variable-font-face-02.html [ Failure ]