(Set)IsOverridingUserAgent should be called on VisibleItem

ios/chrome/browser/tabs/tab.mm

Index: ios/chrome/browser/tabs/tab.mm
diff --git a/ios/chrome/browser/tabs/tab.mm b/ios/chrome/browser/tabs/tab.mm
index 0e5272141e00bc4956b39cf884dd2d81278f3300..9b36d6d41b753fac0bc249af30b30b03d453c5dd 100644
--- a/ios/chrome/browser/tabs/tab.mm
+++ b/ios/chrome/browser/tabs/tab.mm
@@ -1526,8 +1526,7 @@ - (void)openAppStore:(NSString*)appId {
 }
 
 - (BOOL)useDesktopUserAgent {

[kkhorimoto, 2017/02/22 01:08:40]

The naming of this property is a little confusing, as it's using an imperative
voice (i.e. by the name of the function, I would expect that calling this would
instruct the Tab to begin using a desktop UA).  Can we rename the property as
follows?

@property(nonatomic, readonly, getter=isUsingDesktopUserAgent) BOOL
usingDesktopUserAgent?

- (BOOL)usingDesktopUserAgent {
  return [self navigationManager]->IsUsingDesktopUserAgent();
}

[kkhorimoto, 2017/02/22 01:16:32]

Oops, made a mistake, the implementation would be:

- (BOOL)isUsingDesktopUserAgent {
  return [self navigationManager]->IsUsingDesktopUserAgent();
}

[liaoyuke, 2017/02/22 01:37:43]

yeah, I also don't like the name of the function, but in Android, it's named
getUseDesktopUserAgent
https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chr...

Do we want to follow Android's name or change it to isUsingDesktopUserAgent?

[kkhorimoto, 2017/02/22 01:54:33]

I think we should go ahead and use isUsingDesktopUserAgent.  content// and
Android have been around for longer, which is why we often use them as a
reference for implementation details, but naming/style doesn't have to match
exactly.  Given that renaming this would increase readability of the code, I'd
say this is a good place where we should diverge.

Also, just a heads up...when defining BOOL properties where the getter is
different from the property name, you use the property name when accessing via
dot notation and the getter name when accessing using selector notation:

@property(nonatomic, readonly, getter=isUsingDesktopUserAgent) BOOL
usingDesktopUserAgent;

should be accessed as:
tab.usingDesktopUserAgent OR [tab isUsingDesktopUserAgent]

-  web::NavigationItem* currentItem = self.currentNavigationItem;
-  return currentItem && currentItem->IsOverridingUserAgent();
+  return [self navigationManager]->GetUseDesktopUserAgent();
 }
 
 - (void)enableDesktopUserAgent {