Implement policy::GetOSUsername for POSIX platforms.

Bug: 847265
Change-Id: I332cd4d5bd6996019e132abbd313e765f4e080d3
Reviewed-on: https://chromium-review.googlesource.com/1082662
Reviewed-by: Owen Min <zmin@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Georges Khalil <georgesak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567791}

components/policy/core/common/cloud/cloud_policy_util.cc

@@ -16,6 +16,12 @@
 #include <wincred.h>
 #endif
 
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS) || defined(OS_MACOSX)
+#include <pwd.h>
+#include <sys/types.h>
+#include <unistd.h>
+#endif
+
 #if defined(OS_MACOSX)
 #import <SystemConfiguration/SCDynamicStoreCopySpecific.h>
 #include <stddef.h>
@@ -125,7 +131,13 @@ std::string GetOSArchitecture() {
 }
 
 std::string GetOSUsername() {
-#if defined(OS_WIN)
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS) || defined(OS_MACOSX)
+  struct passwd* creds = getpwuid(getuid());
+  if (!creds || !creds->pw_name)
+    return std::string();
+
+  return creds->pw_name;
+#elif defined(OS_WIN)
   WCHAR username[CREDUI_MAX_USERNAME_LENGTH + 1] = {};
   DWORD username_length = sizeof(username);
 
@@ -139,10 +151,9 @@ std::string GetOSUsername() {
 
   return base::WideToUTF8(username);
 #else
-  // TODO(crbug.com/847265): For now, this is only supported on Windows. Other
-  // platforms will be added when enabled.
+  NOTREACHED();
   return std::string();
-#endif  // OS_WIN
+#endif
 }
 
 }  // namespace policy

components/policy/core/common/cloud/cloud_policy_util.h

@@ -24,7 +24,8 @@ POLICY_EXPORT std::string GetOSPlatform();
 POLICY_EXPORT std::string GetOSArchitecture();
 
 // Returns the username of the logged in user in the OS. This function is
-// platform specific.
+// platform specific. Note that on Windows, this returns the username including
+// the domain, whereas on POSIX, this just returns the username.
 POLICY_EXPORT std::string GetOSUsername();
 
 }  // namespace policy