Mark inspector/debugger/debugger-proto-property.html as flaky.

BUG=313338
TBR=yurys@chromium.org

Review URL: https://codereview.chromium.org/52663004

git-svn-id: svn://svn.chromium.org/blink/trunk@160955 bbb929c8-8fbe-4397-9dbb-9b2b20218538