Extensions: Reduce two string copies in IsBackgroundPageURL().

We can do this without any string copy, so use StringPiece
variant from GURL.

This is interesting because every extension resource fetch goes
through this method.

Bug: None
Change-Id: Ief332c09295b1087f64721e44f026eccc4a49497
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631938Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664028}

extensions/browser/extension_protocols.cc

@@ -521,8 +521,9 @@ void GetSecurityPolicyForURL(const GURL& url,
 }
 
 bool IsBackgroundPageURL(const GURL& url) {
-  std::string path = url.path();
-  return path.size() > 1 && path.substr(1) == kGeneratedBackgroundPageFilename;
+  base::StringPiece path_piece = url.path_piece();
+  return path_piece.size() > 1 &&
+         path_piece.substr(1) == kGeneratedBackgroundPageFilename;
 }
 
 class ExtensionProtocolHandler