Limit alt-codes to Windows; fix Linux Aura Alt+Left/Right.

Linux doesn't support the EF_EXTENDED KeyEvent flag like Windows.
So IsUnicodeKeyCode has a false-positive for [Left|Right]Arrow.
That breaks the [Alt]+[Left|Right]Arrow accelerators for Back/Forward.
(when the Omnibox or another non-content browser textfield has focus)

Limit KeyEvent::IsUnicodeKeyCode functionality to Windows.
Alt-codes (as [Alt]+<num-pad digit>) are not handled on Linux anyway:
http://en.wikipedia.org/wiki/Alt_code#Linux

Cleanup Textfield comment and USE_AURA preprocessor directive blocks.

BUG=339632
TEST=Linux Aura handles the [Alt]+[Left|Right]Arrow Back/Forward accelerators when the Omnibox or Find Bar is focused. No [Alt]+<num-pad digit> Unicode alt-code regressions on Windows.
R=sadrul@chromium.org

Review URL: https://codereview.chromium.org/179923008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254614 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254614