Check whether the marking deque overflowed in ProcessEphemeralMarking

Otherwise, we might exit to early when we call ProcessEphemeralMarking
with an empty, overflown deque

BUG=none
R=hpayer@chromium.org,erik.corry@gmail.com
LOG=n

Review URL: https://codereview.chromium.org/777643002

Cr-Commit-Position: refs/heads/master@{#25660}