Fix unused variable in DeferredLegacyStyleInterpolationTest

This fixes a build failure caused by https://codereview.chromium.org/292173009/
due to an unused local variable in release.

NOTRY=TRUE

Review URL: https://codereview.chromium.org/306483012

git-svn-id: svn://svn.chromium.org/blink/trunk@174919 bbb929c8-8fbe-4397-9dbb-9b2b20218538