[ios] Do not copy JS evaluation handlers before passing to WKWebView.

ios/web/web_state/ui/web_view_js_utils.mm

Index: ios/web/web_state/ui/web_view_js_utils.mm
diff --git a/ios/web/web_state/ui/web_view_js_utils.mm b/ios/web/web_state/ui/web_view_js_utils.mm
index 8f7b202b044bd60289af98e906e276c243cbe1c2..fab245f5ad53b55ac61e96f412cc0ed20e586117 100644
--- a/ios/web/web_state/ui/web_view_js_utils.mm
+++ b/ios/web/web_state/ui/web_view_js_utils.mm
@@ -56,22 +56,14 @@ void EvaluateJavaScript(WKWebView* web_view,
                         NSString* script,
                         JavaScriptCompletion completion_handler) {
   DCHECK([script length]);
-  // __block qualifier ensures that web_view_completion_handler is correctly
-  // captured by itself.
-  __block void (^web_view_completion_handler)(id, NSError*) = nil;
-  // Do not create a web_view_completion_handler if no |handler| is passed to
-  // this function. This results in no creation of an unnecessary block.
+  void (^web_view_completion_handler)(id, NSError*) = nil;
+  // Do not create a web_view_completion_handler if no |completion_handler| is
+  // passed to this function. WKWebView flushes it's completion handlers on
+  // deallocation, which will be inefficient use of resources.

[shreyasv1, 2015/11/10 21:34:04]

It's not clear what resource usage is being referred to here.

(Obviously) installing a completion handler when not needed is superfluous.
We can all agree to that.

This statement make it so that it also has to do with WKWebView deallocating
completion handlers on deallocation.
Also, I have never heard the term "flush" used with a completion handler. Do you
mean deallocated or purged?

[Eugene But (OOO till 7-30), 2015/11/10 21:44:05]

It's not only superfluous, it is actually more work for web view.
No, I mean flushed (they are called with error before deallocation). Removed
this work from the comments.

Updated description. Please let me know if's still unclear why we prefer to pass
nil handlers.

   if (completion_handler) {
-    // WKWebView crashes on deallocation when it flushes scripts that did not
-    // finish the execution but have |completion_handler|. Passing
-    // |completion_handler| ownership to the block itself and keeping it alive
-    // until it's executed fixes the crash. No memory leak is expected since
-    // |completion_handler| is always executed even if WKWebView is deallocated.
-    // https://bugs.webkit.org/show_bug.cgi?id=140203
-    web_view_completion_handler = [^(id result, NSError* error) {
+    web_view_completion_handler = ^(id result, NSError* error) {
       completion_handler(UIResultFromWKResult(result), error);
-      [web_view_completion_handler autorelease];
-    } copy];
+    };
   }
   [web_view evaluateJavaScript:script
              completionHandler:web_view_completion_handler];