DevTools: [CodeMirror] add support for camel case movements.

Camel-case movement allows movement between
subwords of camel-case-written words or phrases.

This patch adds support for camel-case movement to the DevTools editor.
The camel-case movement is done via Alt-Left/Alt-Right on Win and Linux,
and via Ctrl-Left/Ctrl-Right on Mac.

Review URL: https://codereview.chromium.org/357483007

git-svn-id: svn://svn.chromium.org/blink/trunk@177099 bbb929c8-8fbe-4397-9dbb-9b2b20218538