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This CL changes device::SerialConnection to hold a base::WeakPtrFactory
member rather than inherit base::SupportsWeakPtr<> to avoid the subtle
use-after-destroy issue:
1.
SerialConnection dtor starts, destroyes its'
device::mojom::SerialIoHandlerPtr |io_handler_| member.
2.
The SerialIoHandlerPtr dtor triggers some one callback XXX wrapped by
ScopedCallbackRunner to run.
3.
The callback XXX was actually bound with a weak ptr of SerialConnection,
at this time point the parent class SupportsWeakPtr's dtor has not been
triggered yet so that the weak ptr is still in a valid state, although
we are already inside the execution context of SerialConnection dtor.

A base::WeakPtrFactory member at the end of member list will be
destroyed immediatelly once entering SerialConnection dtor, thus in the
above step 3 the bound weak ptr should be identified as invalid so that
callback XXX won't be called at all. This is the expected behavior.

BUG=757756

Change-Id: Ib73ac55dcfba250c4f8b96fb9e3d3e85490ea9a0
Reviewed-on: https://chromium-review.googlesource.com/626197Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#496526}