Disk cache: Extend the internal buffering performed by each entry

to cover external files.

We now keep a variable-size buffer and use it even after we
know that the data is not going to be stored by a block-file.
The backend keeps track of the total memory used by all entries
and prevents that value from going over a max value that
depends on the total memory available.

This CL removes the tests that were checking the synchronous
operation of sparse IO because that model is no longer supported
by the public API, and this CL would add complexity to them
(they fail due to thread safety concerns).

BUG=6626
TEST=net_unittests

Review URL: http://codereview.chromium.org/3167020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57082 0039d316-1c4b-4281-b951-d872f2087c98