Don't emit an UMA error for attachment brokering a MACH_PORT_NULL.
Don't emit an UMA error for attachment brokering a MACH_PORT_NULL.

BUG=

Review URL: https://codereview.chromium.org/1899133008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388840}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk