Fixed NavigationItemImpl copy constructor.

Make mutableCopy of http_request_headers_ to avoid crashes in
AddHttpRequestHeaders and RemoveHttpRequestHeaderForKey.

Bug: 798070
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Iaca5eb950f79b5936a75523392322036a37158a5
Reviewed-on: https://chromium-review.googlesource.com/846433
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526775}

ios/web/navigation/navigation_item_impl.mm

@@ -66,7 +66,7 @@ NavigationItemImpl::NavigationItemImpl(const NavigationItemImpl& item)
       ssl_(item.ssl_),
       timestamp_(item.timestamp_),
       user_agent_type_(item.user_agent_type_),
-      http_request_headers_([item.http_request_headers_ copy]),
+      http_request_headers_([item.http_request_headers_ mutableCopy]),
       serialized_state_object_([item.serialized_state_object_ copy]),
       is_created_from_push_state_(item.is_created_from_push_state_),
       has_state_been_replaced_(item.has_state_been_replaced_),

ios/web/navigation/navigation_item_impl_unittest.mm

@@ -90,6 +90,9 @@ TEST_F(NavigationItemTest, Copy) {
   EXPECT_NSEQ([postData0 dataUsingEncoding:NSUTF8StringEncoding],
               copy.GetPostData());
   EXPECT_NSEQ(state0, copy.GetSerializedStateObject());
+
+  // Ensure that HTTP headers are still mutable after the copying.
+  copy.AddHttpRequestHeaders(@{});
 }
 
 // Tests whether |NavigationItem::AddHttpRequestHeaders()| adds the passed