[Extensions] Update PrefHashBrowserTestBase to manifest v2

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

Bug: 816679

Change-Id: Idfa6c27efac318842ad5169370e2073d02edb615
Reviewed-on: https://chromium-review.googlesource.com/1014561Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553313}

chrome/browser/prefs/tracked/pref_hash_browsertest.cc

@@ -202,8 +202,6 @@ class PrefHashBrowserTestBase
       : protection_level_(GetProtectionLevelFromTrialGroup(GetParam())) {
   }
 
-  bool ShouldAllowLegacyExtensionManifests() override { return true; }
-
   void SetUpCommandLine(base::CommandLine* command_line) override {
     ExtensionBrowserTest::SetUpCommandLine(command_line);
     EXPECT_FALSE(command_line->HasSwitch(switches::kForceFieldTrials));