Changes mojo::WaitSet and mojo::SyncHandleRegistry to support waiting on
arbitrary user events concurrently with Mojo handle state changes.

Reverts IPC::SyncChannel to use WaitableEvent for forced wakeups in lieu
of signaling with one-off message pipes (i.e. "MojoEvents", which have
been removed here as well.)

Also uses WaitableEvent for blocking in the bindings' MultiplexRouter and
IPC Channel's internal equivalent.

BUG=698460

Review-Url: https://codereview.chromium.org/2754143005
Cr-Commit-Position: refs/heads/master@{#459486}