Fix sticky keys crash when handling synthesized events.

BUG=320158
TEST=new unit test

Review URL: https://codereview.chromium.org/79453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236729