(Set)IsOverridingUserAgent should be called on VisibleItem

ios/chrome/browser/tabs/tab.h

Index: ios/chrome/browser/tabs/tab.h
diff --git a/ios/chrome/browser/tabs/tab.h b/ios/chrome/browser/tabs/tab.h
index 163c827457e667e8de1a2aa9fe11a39a6add8bf8..b1e01948ace47ce23960453185a02924bcfcdb06 100644
--- a/ios/chrome/browser/tabs/tab.h
+++ b/ios/chrome/browser/tabs/tab.h
@@ -133,7 +133,11 @@ extern NSString* const kProxyPassthroughHeaderValue;
 @property(nonatomic, assign) id<TabHeadersDelegate> tabHeadersDelegate;
 @property(nonatomic, assign) id<TabSnapshottingDelegate>
     tabSnapshottingDelegate;
-@property(nonatomic, readonly) BOOL useDesktopUserAgent;
+
+// Whether or not desktop user agent is used for the currently visible page.
+@property(nonatomic, readonly, getter=isUsingDesktopUserAgent)

[Eugene But (OOO till 7-30), 2017/02/23 02:36:59]

Sorry. Did not notice before. Drop isUsingDesktopUserAgent, it's not correct
name for |usesDesktopUserAgent| property.

[liaoyuke, 2017/02/23 17:50:42]

Hey Kurt, FYI, we drop |isUsingDesktopUserAgent| because it's incorrect as using
v.s. uses, which would break KVC.

+    BOOL usesDesktopUserAgent;
+
 @property(nonatomic, assign) id<StoreKitLauncher> storeKitLauncher;
 @property(nonatomic, assign) id<FullScreenControllerDelegate>
     fullScreenControllerDelegate;