Fix condition of FileReader::hasPendingActivity

Before this patch, |hasPendingActiviy| checked |m_loadingState| to check if it is stopped.
However, when called |abort()|, FileReader schedules a delayed task to call |doAbort()|, while setting |m_loadingState| to |LoadingStateAborted|. This may cause the FileReader to go away when |doAbort()| is called.

This patch avoids the issue by having |hasPendingActivity| look at |m_state| instead.

BUG=393425

Review URL: https://codereview.chromium.org/389193002

git-svn-id: svn://svn.chromium.org/blink/trunk@178019 bbb929c8-8fbe-4397-9dbb-9b2b20218538