Introduce MockHttpCache::backend() and move a few callers to it.

Introduce MockHttpCache::backend() and move a few callers to it.

The MockHttpCache::OpenBackendEntry and CreateBackendEntry functions may
have been making invalid virtual calls if the disk cache backend was not a
MockDiskCache. Fixes HttpCache.RangeGET_LargeValues under -fsanitize=cfi-vptr.

BUG=457523
TEST=net_unittests
R=rdsmith@chromium.org

Committed: https://crrev.com/642379648bdca92a835fb587dd987310161d26f7
Cr-Commit-Position: refs/heads/master@{#317970}

[Randy Smith (Not in Mondays), 2015-02-24 18:02:32]

rdsmith@chromium.org changed reviewers:
+ rvargas@chromium.org
- rdsmith@chromium.org

[Randy Smith (Not in Mondays), 2015-02-24 18:02:32]

Sorry, while this is a simple change, I don't have an understanding of the
requirements of the callers.  Lateralling to Ricardo, who wrote the original
code.

[rvargas (doing something else), 2015-02-24 23:03:52]

lgtm

[pcc1, 2015-02-24 23:04:22]

The CQ bit was checked by pcc@chromium.org

[commit-bot: I haz the power, 2015-02-24 23:05:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956433002/1

[commit-bot: I haz the power, 2015-02-24 23:23:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-24 23:23:56]

Try jobs failed on following builders:
  ios_rel_device_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ng...)
  ios_rel_device_ninja_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[pcc1, 2015-02-24 23:25:07]

The CQ bit was checked by pcc@chromium.org

[commit-bot: I haz the power, 2015-02-24 23:25:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956433002/1

[commit-bot: I haz the power, 2015-02-24 23:42:01]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-24 23:42:02]

Try jobs failed on following builders:
  ios_rel_device_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ng...)
  ios_rel_device_ninja_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[pcc1, 2015-02-25 03:29:31]

The CQ bit was checked by pcc@chromium.org

[commit-bot: I haz the power, 2015-02-25 03:29:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956433002/1

[commit-bot: I haz the power, 2015-02-25 03:33:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-25 03:33:29]

Patchset 1 (id:??) landed as
https://crrev.com/642379648bdca92a835fb587dd987310161d26f7
Cr-Commit-Position: refs/heads/master@{#317970}