Change deprecation message of eventSender to warning

Replacing eventSender by pointerActionSequence is not trivial. The deprecation
message should be a warning, so that CLs with eventSender can be at least
uploaded for reference or discussion purposes.

BUG=n/a

Review-Url: https://codereview.chromium.org/2872133011
Cr-Commit-Position: refs/heads/master@{#471159}
1 file changed