Allow compilation on recent versions of FreeBSD, which have libexecinfo in the base system but return size_t instead of int for backtrace()

This is part of making GN bootstrap on FreeBSD.

Tested on FreeBSD 8.4-i386 and FreeBSD 10.0-amd64

BUG=

Review URL: https://codereview.chromium.org/178193018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256484 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256484