Re-work the 'external snapshot' related build rules.

Re-work the 'external snapshot' related build rules.
This prepares for re-landing crrev.com/956373002

This pulls all decision about the snapshot [no|internal|external] into one rule. Previously, this logic was in separate places and not /quite/ the same, which causes build problems.

BUG=

Committed: https://crrev.com/ee9c738929f2b3195e9486d9e0ec038f031dfce1
Cr-Commit-Position: refs/heads/master@{#27523}

[vogelheim, 2015-03-27 11:36:46]

vogelheim@chromium.org changed reviewers:
+ machenbach@chromium.org, rmcilroy@chromium.org

[rmcilroy, 2015-03-27 16:22:53]

lgtm.

[vogelheim, 2015-03-30 14:53:03]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-03-30 14:53:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1016603004/40001

[commit-bot: I haz the power, 2015-03-30 14:54:25]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-03-30 14:54:36]

Patchset 3 (id:??) landed as
https://crrev.com/ee9c738929f2b3195e9486d9e0ec038f031dfce1
Cr-Commit-Position: refs/heads/master@{#27523}