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This adds members to mojo::Event that are needed to construct
a blink::WebInputEvent, as well as the boilerplate to carry
those members on a ui::KeyEvent object, because the events
aren't sent directly from the native event source to html
viewer, but pass through multiple systems.

BUG=403514

Review URL: https://codereview.chromium.org/492863002

Cr-Commit-Position: refs/heads/master@{#291301}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291301 0039d316-1c4b-4281-b951-d872f2087c98