GN: Make :shared_memory_support a public_dep of //media.
GN: Make :shared_memory_support a public_dep of //media.

It looks like :shared_memory_support is intended to be considered a "part of"
//media:media, and so should be exposed as a public_dep. This resolves 36
errors in `gn check out/Default //content/renderer/*`.

Alternatively, clients could be made to have an explicit dependency on
:shared_memory_support.

Review URL: https://codereview.chromium.org/1564833002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367916}

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