Have PNaCl use debug pexe URL instead of stripped pexe w/ kEnableNaClDebug

Debug metadata is not part of the PNaCl stable bitcode format
so is stripped before deployment. In order to make debugging
easier (no separate translation), allow a separate debug URL
in the NMF file which can point to the unstripped
LLVM-formatted pexe which should still have debug metadata.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3765
TEST=browser_tests --gtest_filter=*PnaclDebugURL

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=254212

R=bradnelson@chromium.org, bradnelson@google.com, dmichael@chromium.org

Review URL: https://codereview.chromium.org/181153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255499 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255499