Add code field speced for DOM KeyEvent into virtual key events from the soft-keyboard.
BUG=324899

Review URL: https://codereview.chromium.org/99973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238716 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238716