ipc: Add basic memory usage tests for attachment brokering.
ipc: Add basic memory usage tests for attachment brokering.

I added two tests for memory leaks in the attachment brokering process. The
first checks memory usage after brokering a single, large, shared memory object.
The second checks memory usage after brokering a thousand, smaller memory
objects.

BUG=557387

Review URL: https://codereview.chromium.org/1465623002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360935}

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