Switching NaCl IRT to be built inside the chrome build.

Fifth attempt to land!

Switching IRT to be built inside the chrome build. Dropping the IRT download
step from the DEPS. Step3 of a many step plan to switch where ppapi + irt
are built.
Dropping download_nacl_irt because we no longer rely on a prebuilt copy.
Dropping irt download drop source tarball (assume people using it will have
to download / build their own nacl toolchain).

Old Review URL: http://codereview.chromium.org/7669058

R=noelallen@google.com
BUG=http://code.google.com/p/chromium/issues/detail?id=93520
TEST=None
Review URL: http://codereview.chromium.org/7693028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@98106 0039d316-1c4b-4281-b951-d872f2087c98

• DEPS[diff]
• build/download_nacl_irt.py[Deleted - diff]
• chrome/build_nacl_irt.py[Added - diff]
• chrome/nacl.gypi[diff]
• tools/export_tarball/export_tarball.py[diff]