[ios] Cleaned up -[CRWWebController prepareForGoBack].

Removed injectWindowID call and |wasShowingInterstitial| local variable.
|injectWindowID| is leftover from UIWebView world where it was necessary
to check for URL changes and to check for URL changes it was necessary
to inject early scripts:
https://codereview.chromium.org/1892483007/diff/20001/ios/web/web_state/ui/crw_web_controller.mm

BUG=628832

Review-Url: https://codereview.chromium.org/2165103002
Cr-Commit-Position: refs/heads/master@{#407813}

ios/web/web_state/ui/crw_web_controller.mm

@@ -2219,17 +2219,8 @@ - (void)loadCancelled {
 }
 
 - (void)prepareForGoBack {
-  // Make sure any transitions that may have occurred have been seen and acted
-  // on by the CRWWebController, so the history stack and state of the
-  // CRWWebController is 100% up to date before the stack navigation starts.
-  if (_webView) {
-    [self injectWindowID];
-  }
-
-  bool wasShowingInterstitial = _webStateImpl->IsShowingWebInterstitial();
-
   // Before changing the current session history entry, record the tab state.
-  if (!wasShowingInterstitial) {
+  if (!_webStateImpl->IsShowingWebInterstitial()) {
     [self recordStateInHistory];
   }
 }