Removing using base::StringPiece from Data_reduction_proxy

It's an anti-pattern to have "using" for a namespace that contains
methods that the code depends on.

Bug: 678368
Change-Id: Icbfbceb35c2384fd6ae20c187d579f1df7913a90
Reviewed-on: https://chromium-review.googlesource.com/803619Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521470}

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc

@@ -23,8 +23,6 @@
 #include "net/http/http_util.h"
 #include "net/url_request/url_request.h"
 
-using base::StringPiece;
-
 namespace {
 
 const char kChromeProxyHeader[] = "chrome-proxy";
@@ -290,7 +288,8 @@ bool ParseHeadersAndSetBypassDuration(const net::HttpResponseHeaders& headers,
     if (StartsWithActionPrefix(value, action_prefix)) {
       int64_t seconds;
       if (!base::StringToInt64(
-              StringPiece(value).substr(action_prefix.size() + 1), &seconds) ||
+              base::StringPiece(value).substr(action_prefix.size() + 1),
+              &seconds) ||
           seconds < 0) {
         continue;  // In case there is a well formed instruction.
       }