Don't GC message pipes that have an in-flight request
Don't GC message pipes that have an in-flight request

In mojom, there's a notion of a request/response pair. This CL teaches the JS
bindings not to GC a message pipe after a request has been issued but before
the response has been received.

R=hansmuller@chromium.org

Review URL: https://codereview.chromium.org/696373003

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