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This patch introduces a BackgroundSyncProvider for use by the Blink
BackgroundSync module, as well as a proxy object which is used when calling the
provider from off of the main thread. The code in BlinkPlatformImpl will
dispatch to the appropriate object depending on the running thread.

This patch depends on the Mojo service definitions in
https://codereview.chromium.org/1106193002

BUG=479251

Review URL: https://codereview.chromium.org/1104283002

Cr-Commit-Position: refs/heads/master@{#329417}