Move SyncPointManager into gpu/command_buffer/service

The SyncPointManager doesn't have any other dependencies and we'd like to reuse
it in Mojo. This CL moves it from content to gpu to make it easier to reuse.

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Review URL: https://codereview.chromium.org/735823002

Cr-Commit-Position: refs/heads/master@{#304698}
10 files changed