[fsp] Fix crash when reading files.

[fsp] Fix crash when reading files.

Before, the net::IOBuffer was being passed as a pointer, from the
FileStreamReader::Read to operations::ReadFile. However, there are PostTask
invocations on the way. When FileStreamReader is deleted after a PostTask is
fired from FileStreamReader::Read, and the operation is not completed, then
the net::IOBuffer will become invalid. When the operation is completed,
operation::ReadFile would cause a segmentation fault.

Since net::IOBuffer is ref counted, we should pass it as ref counted object
to be sure, that the buffer is always valid.

This patch migrates from passing IOBuffer as a pointer, to scoped_refptr.

TEST=Tested manually, that crashing doesn't occur anymore.
BUG=248427

Review URL: https://codereview.chromium.org/301973007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274174 0039d316-1c4b-4281-b951-d872f2087c98