Gracefully handle FileReader() read operations when in a detached state.

Follow up r178274 and have FileReader handle the case when it becomes
detached from its document and its frame. Read operations can no longer
go ahead when that is the case, as the throttling controller isn't
available.

R=haraken@chromium.org,horo@chromium.org,nhiroki@chromium.org,kinuko@chromium.org
BUG=394828

Review URL: https://codereview.chromium.org/399863002

git-svn-id: svn://svn.chromium.org/blink/trunk@178426 bbb929c8-8fbe-4397-9dbb-9b2b20218538