An earilier bug-workaround kept the media in a loading state when
a read operation was pending, but didn't always transition back to
idle when the read operation was done. This CL fixes the problem
by making sure that we check the loading state after read callbacks.

These extra calls to check the loading state can in some cases cause
us to miss progress callbacks. We fix this by allowing in-flight
progress callbacks to complete even if reader_ has been destroyed.

BUG=662615

Review-Url: https://codereview.chromium.org/2481673004
Cr-Commit-Position: refs/heads/master@{#430860}