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Depending on the libc used with your NaCl plugins, if the VideoEncoder
proxy receives a EncodeReply message after it has been closed, the
plugin will crash (newlib) or enter an infinite loop (glibc) because
it tries to remove the end() iterator from a std::map.

BUG=455409
TEST=run ppapi/examples/video_encode multiple times and verify the test keeps recording new videos

Review URL: https://codereview.chromium.org/1138483002

Cr-Commit-Position: refs/heads/master@{#328997}