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This makes it so that attaching gdb to a nacl_helper process, or using gdb
on a core dump from such a process, automatically finds the symbols of the
nacl_helper executable and all the shared libraries it uses.  The theory of
operation and the details are explained in comments in the code.

BUG= http://code.google.com/p/chromium/issues/detail?id=103436
TEST= nacl still works, gdb attach on nacl_helper process, or gdb on core file from one, find symbols

R=mseaborn@chromium.org,bradchen@google.com

Review URL: http://codereview.chromium.org/8491060

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110120 0039d316-1c4b-4281-b951-d872f2087c98