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InitInternal() and ReadInternal() were used to share the same implementation with UploadFileElementReaderSync.
There is no need to maintain them since UploadFileElementReaderSync is gone.
Abandon these unneeded functions and use FileStream in an asynchronous way.

BUG=294797
TEST=net_unittests

Review URL: https://codereview.chromium.org/139253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245509 0039d316-1c4b-4281-b951-d872f2087c98