DevTools: eliminate eventSender usage in text editor tests

This patch removes eventSender usage in text editor tests as this
proved to be flaky. Instead, it introduces InspectorTest.fakeKeyEvent
method which emulates key events in codemirror via js.

Review URL: https://codereview.chromium.org/146663006

git-svn-id: svn://svn.chromium.org/blink/trunk@165847 bbb929c8-8fbe-4397-9dbb-9b2b20218538