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When testing with Nearby Share we were seeing crashes during Bluetooth
socket sending. When ever the crash happened we would see the same
write_request getting processed twice. When the write request gets to
the underlying socket implementation we would hit this CHECK: [1]. This
was happening because SendFrontWriteRequest can get called again while
write request is still pending (a call is always queued after write
completion regardless of the queue state) and it grabs the front of the
queue. The queue item for a write request does not get popped off until
the transfer is complete which allows the same request to get processed
twice (depending on timing).

The solution here is to remove the pending request from the queue right
away and store while it is in progress. In SendFrontWriteRequest we check
if there is already a pending request and exit early.
SendFrontWriteRequest always gets re-queued when the pending write
request completes so we always keep processing.

Tested on device with transfers between Android and ChromeOS.

[1] https://source.chromium.org/chromium/chromium/src/+/master:net/socket/socket_posix.cc;l=331

Fixed: 1121747
Change-Id: I67b69f735798f832aecdd62b6697ecef8d99cfaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429863Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#810749}