Updater: Get values for headers from update_client::NetworkFetcher.

Bug: 1020650
Change-Id: Ie9f32ea00acf8f7c3269da0916866757e8317b03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2267846Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Mila Green <milagreen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782590}

chrome/updater/mac/net/network_fetcher.mm

@@ -23,8 +23,10 @@
 #import "net/base/mac/url_conversions.h"
 #include "url/gurl.h"
 
-const NSString* kHeaderEtag = @"ETag";
-const NSString* kHeaderXRetryAfter = @"X-Retry-After";
+const NSString* kHeaderEtag =
+    base::SysUTF8ToNSString(update_client::NetworkFetcher::kHeaderEtag);
+const NSString* kHeaderXRetryAfter =
+    base::SysUTF8ToNSString(update_client::NetworkFetcher::kHeaderXRetryAfter);
 
 using ResponseStartedCallback =
     update_client::NetworkFetcher::ResponseStartedCallback;