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EventFromNative() returns null when given a system defined event, so filter them
out when monitoring the event stream.
When running the experimental MacViews browser, I was seeing an event
with type=SysDefined subtype=7 (apparently it means that a mouse button
state has changed).

BUG=425229

Review URL: https://codereview.chromium.org/981693002

Cr-Commit-Position: refs/heads/master@{#319142}