Mark crbug.com/339238 inspector/layer-tree-model.html as flaky.

BUG=339238
TBR=caseq

Review URL: https://codereview.chromium.org/149893002

git-svn-id: svn://svn.chromium.org/blink/trunk@166057 bbb929c8-8fbe-4397-9dbb-9b2b20218538