Issue #1484 - Remove unused argument from 'WBPassthruSink::OnStart'

Issue #1484 - Remove unused argument from 'WBPassthruSink::OnStart'

The argument 'handled' to 'WBPassthruSink::OnStart' was not referenced.
Removing this argument brings this member function back to the same signature
as defined in its base class 'PassthroughAPP::CInternetProtocolSinkWithSP'.

The only reference to this argument as a local variable 'handled' in
'PassthroughAPP::CustomSinkStartPolicy::OnStart'. Removing this variable
brings the code for this function closer to the original PassthroughAPP code.

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This review is a dependency of the following one:
    https://codereview.adblockplus.org/29331590/

[Eric, 2015-11-30 13:57:07]

First of two reviews to fix #1484. This one should be obvious.

[sergei, 2015-11-30 14:20:32]

LGTM

[Oleksandr, 2015-12-01 23:08:19]

LGTM