**STILLBAKING** Decouple disk cache tests from backends.

**STILLBAKING** Decouple disk cache tests from backends.

The existing disk_cache entry and backend tests are mostly refactored
into value parameterized tests in GoogleTest, which are then instantiated
for each backend built into your net lib.

The instantiation value is a concrete instance of a backend traits
class, which allows each backend to customize how it interacts with
each test.

The result is a suite of tests that are independent of which specific
backends are being built with the net lib. With this change, chrome
can now be built without net unittests.

This CL is downstream of https://codereview.chromium.org/117533003/

BUG=331062

[gavinp, 2014-01-22 00:48:12]

rebase

[gavinp, 2014-01-22 18:48:23]

remove phantom entry_unittest.cc