Cast find_in_page.js return value from id to NSString.

Currently JsFindinpageManager passes the result of find_in_page.js to
base::SysNSStringToUTF8 with "id" type. This CL casts it to NSString
to prevent it from crashing the APP when the result is not NSString.

Bug: 924441
Change-Id: Ib02e80c09ef8c3af9f877f0c70cf1361fc019dd5
Reviewed-on: https://chromium-review.googlesource.com/c/1433754
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#626028}(cherry picked from commit 831dfd6)
Reviewed-on: https://chromium-review.googlesource.com/c/1445038
Reviewed-by: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#61}
Cr-Branched-From: e510299-refs/heads/master@{#625896}

Author: Yi Su

ios/chrome/browser/find_in_page/js_findinpage_manager.mm

@@ -169,11 +169,12 @@ - (void)disableWithCompletionHandler:(ProceduralBlock)completionHandler {
 #pragma mark FindInPageEntry
 
 - (BOOL)processFindInPageResult:(id)result scrollPosition:(CGPoint*)point {
-  if (!result)
+  NSString* result_str = base::mac::ObjCCastStrict<NSString>(result);
+  if (!result_str)
     return NO;
 
   // Parse JSONs.
-  std::string json = base::SysNSStringToUTF8(result);
+  std::string json = base::SysNSStringToUTF8(result_str);
   std::unique_ptr<base::Value> root(base::JSONReader::Read(json, false));
   if (!root.get())
     return YES;