DeviceUtils: Write checksums to cache even when not reading them

DeviceUtils: Write checksums to cache even when not reading them

This allows clients to use DumpCacheData() even when
enable_device_files_cache=False.

Useful for when enable_device_files_cache is set by a flag (so that it doesn't
require two runs to be effective)

BUG=540857
Committed: https://crrev.com/ed679db4616176ad6245f2bc71be2b068ff01127
Cr-Commit-Position: refs/heads/master@{#353066}

[agrieve, 2015-10-08 01:20:17]

agrieve@chromium.org changed reviewers:
+ jbudorick@chromium.org

[agrieve, 2015-10-08 01:20:31]

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

[commit-bot: I haz the power, 2015-10-08 01:21:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386353005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386353005/1

[commit-bot: I haz the power, 2015-10-08 02:26:22]

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

[commit-bot: I haz the power, 2015-10-08 02:26:23]

Dry run: This issue passed the CQ dry run.

[jbudorick, 2015-10-08 14:28:02]

lgtm

[agrieve, 2015-10-08 15:46:11]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-10-08 15:47:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386353005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386353005/1

[commit-bot: I haz the power, 2015-10-08 15:53:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 15:54:04]

Patchset 1 (id:??) landed as
https://crrev.com/ed679db4616176ad6245f2bc71be2b068ff01127
Cr-Commit-Position: refs/heads/master@{#353066}