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Ensure Keypad Events on Mac have the correct keycode associated with
them by avoiding the character conversion code and doing the
correct check based on the mac KeyCode value.

For example '0' as a Keypad character was producing the exact
same keycode as '0' as an alpha numeric key because it was
compared via the text conversion function first.

This code is taken from the WebInputEventFactory.mm in blink.

BUG=493833

Review URL: https://codereview.chromium.org/1331383004

Cr-Commit-Position: refs/heads/master@{#348798}