ipc: Fix unused variable error when using IPC_HANDLER_GENERIC.
ipc: Fix unused variable error when using IPC_HANDLER_GENERIC.

If we use only IPC_HANDLER_GENERIC inside an IPC_BEGIN_MESSAGE_MAP block
we get an unused variable compile error for the extra param that's used
in IPC_BEGIN_MESSAGE_MAP_WITH_PARAM. This CL suppresses that error.

BUG=NONE

Review URL: https://codereview.chromium.org/1667353002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#373714}

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