[SimpleCache] Prefetch stream 1 so that the first read is faster

[SimpleCache -- INCOMPLETE] Prefetch stream 1 so that the first read is faster

90% of cache reads are < 32KB. If we prefetch stream 1 when opening an entry,
then for cache hits, we'd make 90% of the entry's reads synchronous. There
is a slight additional cost to cache misses but it's just a single read call.

BUG=719979

[jkarlin, 2017-05-09 15:41:26]

Description was changed from

==========
[SimpleCache] Prefetch stream 1 so that the first read is faster

90% of cache reads are < 32KB. If we prefetch stream 1 when opening an entry,
then for cache hits, we'd make 90% of the read operations synchronous. There
is a slight additional cost to cache misses but it's just a single read call.

BUG=719979
==========

to

==========
[SimpleCache -- INCOMPLETE] Prefetch stream 1 so that the first read is faster

90% of cache reads are < 32KB. If we prefetch stream 1 when opening an entry,
then for cache hits, we'd make 90% of the read operations synchronous. There
is a slight additional cost to cache misses but it's just a single read call.

BUG=719979
==========

[jkarlin, 2017-05-09 15:42:32]

Description was changed from

==========
[SimpleCache -- INCOMPLETE] Prefetch stream 1 so that the first read is faster

90% of cache reads are < 32KB. If we prefetch stream 1 when opening an entry,
then for cache hits, we'd make 90% of the read operations synchronous. There
is a slight additional cost to cache misses but it's just a single read call.

BUG=719979
==========

to

==========
[SimpleCache -- INCOMPLETE] Prefetch stream 1 so that the first read is faster

90% of cache reads are < 32KB. If we prefetch stream 1 when opening an entry,
then for cache hits, we'd make 90% of the entry's reads synchronous. There
is a slight additional cost to cache misses but it's just a single read call.

BUG=719979
==========

[jkarlin, 2017-05-09 15:56:45]

https://codereview.chromium.org/2872943002/diff/1/net/disk_cache/simple/simpl...
File net/disk_cache/simple/simple_synchronous_entry.cc (right):

https://codereview.chromium.org/2872943002/diff/1/net/disk_cache/simple/simpl...
net/disk_cache/simple/simple_synchronous_entry.cc:1157: if
(out_entry_stat->sparse_data_size() == 0 &&
The bits in this conditional are completely broken. Mostly cut and pastes that I
intended to hack into something.

[jkarlin, 2017-05-11 13:42:13]

The CQ bit was checked by jkarlin@chromium.org to run a CQ dry run

[jkarlin, 2017-05-11 13:42:24]

The CQ bit was unchecked by jkarlin@chromium.org