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According to the latest spec we should filter non-glyph modifiers if the
produced characters are part of Unicode 'Other, Control' General Category.
e.g. http://www.fileformat.info/info/unicode/category/Cc/list.htm

This patch changes implementation to match spec and fixes issues on
'Dvorak - QWERTY Command' layout.

Spec: https://w3c.github.io/uievents-key/#selecting-key-attribute-values

Bug: 747358
Change-Id: I792b472132ea6deb4213206593136343a2050ccd
Reviewed-on: https://chromium-review.googlesource.com/585638Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Chong Zhang <chongz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491490}