Make sure that _FileInputStream._fillBuffer is not entered again before it finishes.
Make sure that _FileInputStream._fillBuffer is not entered again before it finishes.

Also fix a bug where _fillBuffer did not reset _position correctly.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com//10533110

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@8553 260f80e4-7a28-3924-810f-c04153c831b5

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