Do not create CRWWebUIManager for non-WebUI URLs.

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index 8b17115ffff239e137ea52f7dc8c6d7228fe2a37..9305032c93f290ed5434963b050f892a75c48b93 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -3163,8 +3163,11 @@ registerLoadRequestForURL:(const GURL&)requestURL
 
 - (void)createWebUIForURL:(const GURL&)URL {
   _webStateImpl->CreateWebUI(URL);

[PL, 2017/05/17 00:04:26]

For chrome://crash we would still run CreateWebUI even though we weren't
creating a web UI, is that a problem?

[Eugene But (OOO till 7-30), 2017/05/17 14:49:26]

That should not be a problem. Added explanation comment to the code:
"|CreateWebUI| will do nothing if |URL| is not a WebUI URL.". 

-  _webUIManager.reset(
-      [[CRWWebUIManager alloc] initWithWebState:self.webStateImpl]);
+  bool isWebUIURL = _webStateImpl->HasWebUI();
+  if (isWebUIURL) {
+    _webUIManager.reset(
+        [[CRWWebUIManager alloc] initWithWebState:_webStateImpl]);
+  }
 }
 
 - (void)clearWebUI {
@@ -4118,6 +4121,7 @@ registerLoadRequestForURL:(const GURL&)requestURL
 }
 
 - (void)loadHTML:(NSString*)HTML forURL:(const GURL&)URL {
+  DCHECK(HTML.length);
   // Remove the transient content view.
   [self clearTransientContentView];