DevTools: [CodeMirror] fix context menu for text editor

At present it is impossible to do a right-click for selected text in
text editor: every time one tries to do this - selections gets disabled.

This patch fixes this behavior by catching contextmenu event in a way
coordinated with codemirror text editor.

Review URL: https://codereview.chromium.org/247013004

git-svn-id: svn://svn.chromium.org/blink/trunk@172362 bbb929c8-8fbe-4397-9dbb-9b2b20218538