Reland - Create a database for NaCl validation caching that is shared between processes.

Originally reverted in 129077 due to perf regression. Followup commit will fix up expectations.
http://chromegw.corp.google.com/i/chromium/builders/Linux%20x64/builds/25780

This change primarily entails creating a SyncChannel between sel_ldr and the
browser.  Queries to the database could be made from any thread inside sel_ldr,
so the query mechanism needs to be thread safe.

This feature is currently disabled by default, and requires an environment
variable to enable.  A few changes need to be made before this features is safe
and can be enabled, such as making sure each installation has a unique,
crypographically secure key.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2515
TEST= Run NaCl w/ NACL_VALIDATION_CACHE=1


Review URL: http://codereview.chromium.org/9796006

TBR=ncbray@chromium.org
Review URL: https://chromiumcodereview.appspot.com/9808113

TBR=nduca@google.com
Review URL: https://chromiumcodereview.appspot.com/9860020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129082 0039d316-1c4b-4281-b951-d872f2087c98

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