Fix externs/chrome_extensions.js URL

`curl https://closure-compiler.googlecode.com/git/README` says

"""
Closure Compiler has been moved to github.
Please see:
  https://github.com/google/closure-compiler
"""

closure-compiler.googlecode.com is no more. Also raw.github.com
redirects to raw.githubusercontent.com so just use that.

TEST=Run make. It shouldn't die early on some !DOCTYPE that shouldn't be
in a js file.

R=mgiuca@chromium.org

Review URL: https://codereview.appspot.com/181190043