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The current logic ends up creating a MailboxManager per ContextGroup,
so change this to share the mailbox manager across
InProcessCommandBuffer::Service instances.

This also allows for removing code in Android WebView that deals with
passing around the share group. None of these contexts (child, parent
compositor and video) need client-side share groups since mailbox names
are used.

TBR=tedchoc@chromium.org for content/public

Review URL: https://codereview.chromium.org/540143002

Cr-Commit-Position: refs/heads/master@{#293902}