Allow compilation on recent versions of FreeBSD, which have libexecin…

…fo 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

base/debug/stack_trace_posix.cc

@@ -29,6 +29,7 @@
 #include "base/debug/debugger.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/numerics/safe_conversions.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/strings/string_number_conversions.h"
 
@@ -472,7 +473,7 @@ StackTrace::StackTrace() {
 
   // Though the backtrace API man page does not list any possible negative
   // return values, we take no chance.
-  count_ = std::max(backtrace(trace_, arraysize(trace_)), 0);
+  count_ = base::saturated_cast<size_t>(backtrace(trace_, arraysize(trace_)));
 }
 
 void StackTrace::Print() const {