Read pnacl version from common .json file instead of using dirname.
Read pnacl version from common .json file instead of using dirname.

For about:nacl, it didn't seem too clean to check the
directory structure for the PNaCl version number, and it
required tweaking when we switching from normal CRX'es to
multi-CRX.  This scheme of checking the dirname also doesn't
work with local chrome builds or with ChromeOS.  Instead,
grab the version number from the pnacl_public_pnacl_json.

Depends on: https://codereview.chromium.org/22301014/
and on a new package of PNaCl being pushed out.
However, this should still be tolerant of missing keys
(just won't show the version).

BUG=221381
NOTRY=true
(failed tries unrelated)

Review URL: https://chromiumcodereview.appspot.com/22415017

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@216829 0039d316-1c4b-4281-b951-d872f2087c98

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