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
DevTools: position overlay based on the view position (with potential left/top offsets).

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/141293004

git-svn-id: svn://svn.chromium.org/blink/trunk@165848 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add the user metrics to EditorCommand

Add the UMA histogram metrics counting each executed command as autogenerated ID.
We will add a histogram declaration to chrome/trunk/src/tools/metrics/histograms/histograms.xml in another CL.
BUG=338241

Review URL: https://codereview.chromium.org/148083003

git-svn-id: svn://svn.chromium.org/blink/trunk@165849 bbb929c8-8fbe-4397-9dbb-9b2b20218538