[Cronet] Fix build failure in net_string_util_icu_alternatives_ios.mm

Looks caused by crrev.com/652649

Bug: 820198
Change-Id: Ifa4633140962e1a88097a466370d12f3d39065cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576436
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Auto-Submit: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652835}

net/base/net_string_util_icu_alternatives_ios.mm

@@ -74,7 +74,10 @@ bool ConvertToUTF16WithSubstitutions(base::StringPiece text,
 }
 
 bool ToUpper(base::StringPiece16 str, base::string16* output) {
-  base::ScopedCFTypeRef<CFStringRef> cfstring(base::SysUTF16ToCFStringRef(str));
+  base::string16 str16;
+  str.CopyToString(&str16);
+  base::ScopedCFTypeRef<CFStringRef> cfstring(
+      base::SysUTF16ToCFStringRef(str16));
   base::ScopedCFTypeRef<CFMutableStringRef> mutable_cfstring(
       CFStringCreateMutableCopy(kCFAllocatorDefault, 0, cfstring.get()));
   CFStringUppercase(mutable_cfstring.get(), NULL);