Refine systemInfo.cpu API defintions and provide systemInfo.cpu.get impl for Windows and Linux

base/sys_info_ios.mm

Index: base/sys_info_ios.mm
diff --git a/base/sys_info_ios.mm b/base/sys_info_ios.mm
index 48c7bd944841b1f215e44c0cff71a02c1dff54c4..1f49cacbf27b318d60de2ffcf74960c374483ca7 100644
--- a/base/sys_info_ios.mm
+++ b/base/sys_info_ios.mm
@@ -65,4 +65,10 @@ int64 SysInfo::AmountOfPhysicalMemory() {
   return static_cast<int64>(hostinfo.max_mem);
 }
 
+// static
+std::string SysInfo::CPUModelName() {
+  // TODO(hongbo): Retrieve cpu model name.
+  return "unknown";
+}
+
 }  // namespace base