NaCl: Allow download_nacl_irt.py to be usefully invoked with no arguments
NaCl: Allow download_nacl_irt.py to be usefully invoked with no arguments

Extend the script to read the NaCl revisions/hashes directly from DEPS
in this case.

This makes the script usable in cases where you don't want to run
"gclient runhooks", such as when creating a tarball of the checkout,
in which case it is not desirable to run Gyp and have it generate
makefiles.

Admittedly this is redundant, because it adds a second way of getting
information from DEPS to the download_nacl_irt.py script.

The implementation of EvalDepsFile() is borrowed from:
http://src.chromium.org/viewvc/native_client/trunk/src/native_client/build/chromebinaries.py?revision=4577&view=markup

BUG=http://bugs.gentoo.org/show_bug.cgi?id=366413
TEST=run "python src/build/download_nacl_irt.py"

Review URL: http://codereview.chromium.org/6966010

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@84863 0039d316-1c4b-4281-b951-d872f2087c98

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