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As a part of https://codereview.chromium.org/1774323003, it became clear
that it needed to be possible to call SetClientReady repeatedly, which
some sources didn't support.

Instead of just making that part of the contract, instead fold the two
places where SetClientReady mattered into AddObserver (the first time
where a begin frame could actually be delivered) and remove
SetClientReady from the API.

R=skyostil@chromium.org,sunnyps@chomium.org
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1778853003

Cr-Commit-Position: refs/heads/master@{#381612}