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This makes crash dumping optional for RPH::ShutdownForBadMessage,
and changes the thread-safe bad_message::ReceivedBadMessage to
generate its own crash dump immediately, before potentially posting
a UI thread task.

All other call sites have been updated to opt into crash dumping,
preserving previous behavior.

This ensures that bad message reports from the IO thread have a
useful crash stack.

BUG=652755
R=jam@chromium.org

Review-Url: https://codereview.chromium.org/2387113004
Cr-Commit-Position: refs/heads/master@{#423040}