Implement SysInfo::HardwareModelName() on iOS.

This allows the hardware_class field in UMA to be populated on
iOS (with values like "iPhone9,3"), which will allow us to stop
sending those strings in the cpu_architecture field which is
what's currently being done.

BUG=370104

Review-Url: https://codereview.chromium.org/2860663005
Cr-Commit-Position: refs/heads/master@{#469499}


CrOS-Libchrome-Original-Commit: 666e5eec463d93e15876275e1a8a7538b7117746
diff --git a/base/sys_info.h b/base/sys_info.h
index f4134ea..b507f13 100644
--- a/base/sys_info.h
+++ b/base/sys_info.h
@@ -69,8 +69,9 @@
 
   // Returns a descriptive string for the current machine model or an empty
   // string if the machine model is unknown or an error occured.
-  // e.g. "MacPro1,1" on Mac, or "Nexus 5" on Android. Only implemented on OS X,
-  // Android, and Chrome OS. This returns an empty string on other platforms.
+  // e.g. "MacPro1,1" on Mac, "iPhone9,3" on iOS or "Nexus 5" on Android. Only
+  // implemented on OS X, iOS, Android, and Chrome OS. This returns an empty
+  // string on other platforms.
   static std::string HardwareModelName();
 
   // Returns the name of the host operating system.