DevTools: fix inspector/layers/tracing-layer-tree.html broken by r178040

TBR=yurys,loislo

Review URL: https://codereview.chromium.org/399873002

git-svn-id: svn://svn.chromium.org/blink/trunk@178364 bbb929c8-8fbe-4397-9dbb-9b2b20218538