[NaCl SDK] Fix running "make run_package" with non-default toolchain.

Unfortunately, it is difficult (impossible?) to have command line parameters
modify a packaged app. As a result, we create a file on disk that has the toolchain/config to load. If this file doesn't exist, then we use index.html by default.

BUG=307724
R=sbc@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/27565002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229433 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229433