Move WebInputEvent into public/platform from public/web

Some preliminary work to remove PlatformEvent abstractions.

Rename a constant to pass check-webkit-style.

BUG=625684
TBR=rbyers@chromium.org

Review-Url: https://codereview.chromium.org/2173073003
Cr-Commit-Position: refs/heads/master@{#407186}
20 files changed