We need to empty the deque after EmbedderHeapTracer::TracePrologue, and we don't have to empty the deque after ProcessEphemeralMarking, as the deque should be empty already.

BUG=468240
LOG=no

Review URL: https://codereview.chromium.org/1900783002

Cr-Commit-Position: refs/heads/master@{#35654}