content/gpu: Fix race condition in GpuChannelHost.
content/gpu: Fix race condition in GpuChannelHost.

|context_lock_| should be acquired when modifying |stream_flush_info_|.

BUG=525152
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/1319783004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345768}

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