Add mechanism to get the Tab from the associated WebState.

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 4b3926ec0e35d2d76082cb26c9c3919ee6690ed6..980366ab5e38386f1dd677b8a623a4c048ee7b42 100644
--- a/ios/chrome/browser/tabs/tab.mm
+++ b/ios/chrome/browser/tabs/tab.mm
@@ -88,6 +88,7 @@
 #include "ios/chrome/browser/ssl/ios_security_state_tab_helper.h"
 #import "ios/chrome/browser/storekit_launcher.h"
 #include "ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.h"
+#import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 #import "ios/chrome/browser/tabs/tab_delegate.h"
 #import "ios/chrome/browser/tabs/tab_dialog_delegate.h"
 #import "ios/chrome/browser/tabs/tab_headers_delegate.h"
@@ -535,6 +536,10 @@ void TabInfoBarObserver::OnInfoBarReplaced(infobars::InfoBar* old_infobar,
     webStateObserver_.reset(
         new web::WebStateObserverBridge(webStateImpl_.get(), self));
 
+    // Do not respect |attachTabHelpers| as this tab helper is required for
+    // proper conversion from WebState to Tab.
+    LegacyTabHelper::CreateForWebState(webStateImpl_.get(), self);
+
     [self.webController setDelegate:self];
 
     NSString* sessionID = self.tabId;
@@ -1248,6 +1253,8 @@ void TabInfoBarObserver::OnInfoBarReplaced(infobars::InfoBar* old_infobar,
   // to drive its own destruction.
   base::scoped_nsobject<Tab> kungFuDeathGrip([self retain]);
   [parentTabModel_ didCloseTab:self];  // Inform parent of tab closure.
+
+  LegacyTabHelper::RemoveFromWebState(webStateImpl_.get());

[rohitrao (ping after 24h), 2017/02/08 14:07:17]

Is there any way for code to call LegacyTabHelper::FromWebState() after this
line is run?  I don't want to require null-checking.  If it's basically
impossible for code to run after this call, then that's ok.  (Otherwise we
should consider calling SetTab(nil) instead of removing the helper completely.)

[sdefresne, 2017/02/08 14:45:34]

I've addressed this by introducing a static method
LegacyTabHelper::GetTabForWebState() that does the null check.

   webStateImpl_.reset();
 }