[Extensions] Update StartupHelperBrowserTests to manifest v2

StartupHelperBrowserTests have been updated to manifest v2.
Remove the allowance for manifest v1 extensions.

Bug: 816679
Change-Id: Ib11500da4e22cd74f60193e5fd3d3c0368fb686f
Reviewed-on: https://chromium-review.googlesource.com/1024600Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553863}

chrome/browser/extensions/startup_helper_browsertest.cc

@@ -13,7 +13,6 @@
 #include "chrome/common/chrome_switches.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "extensions/common/extension.h"
-#include "extensions/common/switches.h"
 
 class StartupHelperBrowserTest : public InProcessBrowserTest {
  public:
@@ -23,10 +22,6 @@ class StartupHelperBrowserTest : public InProcessBrowserTest {
   void SetUpCommandLine(base::CommandLine* command_line) override {
     command_line->AppendSwitch(switches::kNoStartupWindow);
 
-    // TODO(devlin): Remove this. See https://crbug.com/816679.
-    command_line->AppendSwitch(
-        extensions::switches::kAllowLegacyExtensionManifests);
-
     PathService::Get(chrome::DIR_TEST_DATA, &test_data_dir_);
     test_data_dir_ = test_data_dir_.AppendASCII("extensions");
     InProcessBrowserTest::SetUpCommandLine(command_line);