[Background Sync] Remove experimental flag from test.

Background Sync is no longer experimental. With this change, the browser
tests will no longer set the "Experimental Web Platform Features" flag.

Bug: 932591
Change-Id: I4468e7510a1dcaf8825e9a39265e04bc303f5f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529051Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641694}

content/browser/background_sync/background_sync_browsertest.cc

@@ -10,7 +10,6 @@
 #include <vector>
 
 #include "base/bind.h"
-#include "base/command_line.h"
 #include "base/macros.h"
 #include "base/run_loop.h"
 #include "base/strings/string_split.h"
@@ -158,13 +157,6 @@ class BackgroundSyncBrowserTest : public ContentBrowserTest {
 
   WebContents* web_contents() { return shell_->web_contents(); }
 
-  void SetUpCommandLine(base::CommandLine* command_line) override {
-    // TODO(jkarlin): Remove this once background sync is no longer
-    // experimental.
-    command_line->AppendSwitch(
-        switches::kEnableExperimentalWebPlatformFeatures);
-  }
-
   void SetUpOnMainThread() override {
     https_server_ = std::make_unique<net::EmbeddedTestServer>(
         net::EmbeddedTestServer::TYPE_HTTPS);