[DevTools] Start/stop of timeline recording should set UI controls disabled/enabled respectively.

Once timeline recording starts, the state of "Capture ..." checkboxes has no effect on the recording behavior.
These controls should be disabled upon the start of recording and reset back upon completion of the recording.

Review URL: https://codereview.chromium.org/361203002

git-svn-id: svn://svn.chromium.org/blink/trunk@177490 bbb929c8-8fbe-4397-9dbb-9b2b20218538