Fetch executable path for Mac in helper

The current implementation for Mac crashes, since /proc/ doesn't exist and thus
the path being searched for. This leads to attempting to create a string of
length 0.

This adds in Mac specific logic for fetching this information.

BUG=chromium:773183

Change-Id: I57bc2530ee79a51c0caad42ec31f640f73c7a7d4
Reviewed-on: https://chromium-review.googlesource.com/740344Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513288}

pdf/pdfium/fuzzers/pdfium_fuzzer_helper.cc

@@ -8,23 +8,27 @@
 
 #include <assert.h>
 #include <limits.h>
+
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 #include <Windows.h>
-#else
+#elif defined(__APPLE__)
+#include <mach-o/dyld.h>
+#else  // Linux
 #include <unistd.h>
-#endif
+#endif  // _WIN32
 
 #include <memory>
 #include <sstream>
 #include <string>
 #include <utility>
 
+#include "base/memory/free_deleter.h"
 #include "third_party/pdfium/public/cpp/fpdf_deleters.h"
 #include "third_party/pdfium/public/fpdf_dataavail.h"
 #include "third_party/pdfium/public/fpdf_text.h"
@@ -78,22 +82,30 @@ FPDF_BOOL Is_Data_Avail(FX_FILEAVAIL* pThis, size_t offset, size_t size) {
 void Add_Segment(FX_DOWNLOADHINTS* pThis, size_t offset, size_t size) {}
 
 std::string ProgramPath() {
-#ifdef _MSC_VER
+  std::string result;
+
+#ifdef _WIN32
   wchar_t wpath[MAX_PATH];
   char path[MAX_PATH];
-  DWORD res = GetModuleFileName(NULL, wpath, MAX_PATH);
-  assert(res != 0);
-  wcstombs(path, wpath, MAX_PATH);
-  return std::string(path, res);
-#else
-  char* path = new char[PATH_MAX + 1];
-  assert(path);
-  ssize_t sz = readlink("/proc/self/exe", path, PATH_MAX);
-  assert(sz > 0);
-  std::string result(path, sz);
-  delete[] path;
-  return result;
+  DWORD len = GetModuleFileNameA(NULL, path, MAX_PATH);
+  if (len != 0)
+    result = std::string(path, len);
+#elif defined(__APPLE__)
+  char path[PATH_MAX];
+  unsigned int len = PATH_MAX;
+  if (!_NSGetExecutablePath(path, &len)) {
+    std::unique_ptr<char, base::FreeDeleter> resolved_path(
+        realpath(path, nullptr));
+    if (resolved_path.get())
+      result = std::string(resolved_path.get());
+  }
+#else  // Linux
+  char path[PATH_MAX];
+  ssize_t len = readlink("/proc/self/exe", path, PATH_MAX);
+  if (len > 0)
+    result = std::string(path, len);
 #endif
+  return result;
 }
 
 }  // namespace