NaCl cleanup: Use validation_cache.h from src/public/

NaCl cleanup: Use validation_cache.h from src/public/

Now that NaClValidationCache is declared in src/public/, we can use
that declaration from the Chromium side and remove a non-public
#include.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=2832
TEST=build

Committed: https://crrev.com/b4e89950984d01c670fde7d062e1ff4ec7ef9d97
Cr-Commit-Position: refs/heads/master@{#316583}

[Mark Seaborn, 2015-02-17 16:15:00]

mseaborn@chromium.org changed reviewers:
+ jvoung@chromium.org

[jvoung (off chromium), 2015-02-17 16:33:27]

lgtm

[Mark Seaborn, 2015-02-17 16:36:07]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2015-02-17 16:37:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/929613002/1

[commit-bot: I haz the power, 2015-02-17 16:40:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-17 16:41:34]

Patchset 1 (id:??) landed as
https://crrev.com/b4e89950984d01c670fde7d062e1ff4ec7ef9d97
Cr-Commit-Position: refs/heads/master@{#316583}