Splitting DidSwap in cc::SwapPromise to WillSwap and DidSwap

Currently DidSwap runs before the swap, but we need a method
that is run after it. DidSwap is also not the best name because
it's called before the swap is done, so we believe WillSwap will
be a better name for it. So DidSwap will be renamed to WillSwap
and we will create a new DidSwap method that is run after the swap.

BUG=659598
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2580493002
Cr-Commit-Position: refs/heads/master@{#438993}
16 files changed