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The execinfo.h header only provides prototypes for backtrace funcs.
These two files don't use those funcs, so the include is unused.
Punt it to make building on non-glibc systems easier.

BUG=None
TEST=trybot for linux/nacl/cros work

Review URL: https://chromiumcodereview.appspot.com/16120002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202666 0039d316-1c4b-4281-b951-d872f2087c98