Blockfile cache: fix long-standing sparse + evict reentrancy problem

Thanks to nedwilliamson@ (on gmail) for an alternative perspective
plus a reduction to make fixing this much easier.

Bug: 826626, 518908, 537063, 802886
Change-Id: Ibfa01416f9a8e7f7b361e4f93b4b6b134728b85f
Reviewed-on: https://chromium-review.googlesource.com/985052
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547103}
void InFlightBackendIO::ReadSparseData(
    EntryImpl* entry,
    int64_t offset,
    net::IOBuffer* buf,
    int buf_len,
    const net::CompletionCallback& callback) {
  scoped_refptr<BackendIO> operation(new BackendIO(this, backend_, callback));
  operation->ReadSparseData(entry, offset, buf, buf_len);
  PostOperation(FROM_HERE, operation.get());
}

void InFlightBackendIO::ReadSparseData(
    EntryImpl* entry,
    int64_t offset,
    net::IOBuffer* buf,
    int buf_len,
    const net::CompletionCallback& callback) {
  scoped_refptr<BackendIO> operation(new BackendIO(this, backend_, callback));
  operation->ReadSparseData(entry, offset, buf, buf_len);
  PostOperation(FROM_HERE, operation.get());
}
