NaCl: Clean up to use the public API for creating validation-cacheable descs

NaCl: Clean up to use the public API for creating validation-cacheable descs

Use the API that's defined in native_client/src/public/.  That is
already used by nacl_listener.cc, but the code in nacl_ipc_adapter.cc
was written earlier, before the API in public/ was added.

NaClDescCreateWithFilePathMetadata() does the same as the code we're
replacing here.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=4073
TEST=browser_tests

Committed: https://crrev.com/88df492682b84068023909c4ce90cd69e246c364
Cr-Commit-Position: refs/heads/master@{#315049}

[Mark Seaborn, 2015-02-06 16:04:15]

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

[teravest, 2015-02-06 16:07:11]

lgtm

[Mark Seaborn, 2015-02-06 16:10:02]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2015-02-06 16:10:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/897363002/1

[commit-bot: I haz the power, 2015-02-06 16:51:43]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-06 16:52:31]

Patchset 1 (id:??) landed as
https://crrev.com/88df492682b84068023909c4ce90cd69e246c364
Cr-Commit-Position: refs/heads/master@{#315049}