removeCallbacksAndMessages() is called on the camera thread handler
before setting it to null to remove all pending runnables. The purpose
is to make sure *OnCameraThread methods are not executed when the
camera is stopped, but this does not seem to work reliably. This CL
resorts to a belt and braces approach and checks that the the handler is
still alive in all *OnCameraThread methods.
BUG=b/29015569
R=sakal@webrtc.org
Review URL: https://codereview.webrtc.org/2028643002 .
Cr-Commit-Position: refs/heads/master@{#12981}
2e646c8729