Fixes document type detection on iOS 9.

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 960077298684222386a48b61319fb94727f7ff32..1f838cd80001c704d0ad8f9cdeef339fed0eb90d 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -3854,4 +3854,18 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
   _externalRequest.reset();
 }
 
+- (web::WebViewDocumentType)documentTypeFromMIMEType:(NSString*)MIMEType {
+  if (!MIMEType.length) {
+    return web::WEB_VIEW_DOCUMENT_TYPE_UNKNOWN;
+  }
+
+  if ([MIMEType isEqualToString:@"text/html"] ||
+      [MIMEType isEqualToString:@"application/xhtml+xml"] ||
+      [MIMEType isEqualToString:@"application/xml"]) {
+    return web::WEB_VIEW_DOCUMENT_TYPE_HTML;
+  }
+
+  return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
+}
+
 @end