[Prerender] Get the prerender mode from Finch field trial.

chrome/browser/extensions/activity_log/activity_log_browsertest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdint.h>
 
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 18 matching lines @@
 // Only the prerender tests are in this file. To add tests for activity
 // logging please see:
 //    chrome/test/data/extensions/api_test/activity_log_private/README
 
 class ActivityLogPrerenderTest : public ExtensionApiTest {
  protected:
   // Make sure the activity log is turned on.
   void SetUpCommandLine(base::CommandLine* command_line) override {
     ExtensionBrowserTest::SetUpCommandLine(command_line);
     command_line->AppendSwitch(switches::kEnableExtensionActivityLogging);
-    command_line->AppendSwitchASCII(switches::kPrerenderMode,

[pasko, 2016/12/01 16:47:58]

if we are removing this, then the tests will need to be adjusted when we turn
off prerender by default, right? is there a way to force the feature state for
the test? I guess that would make switching things on/off reverting/relanding
easier.

If forcing the feature into certain state is hard here, then np.

[droger, 2016/12/01 17:17:40]

I don't have a simple solution unfortunately.
We could still do it by appending command line arguments though, they would just
be more complicated:

--force-fieldtrials=NoStatePrefetchValidation/Prerender
--enable-features="NoStatePrefetch<NoStatePrefetchValidation"
--force-fieldtrial-params=NoStatePrefetchValidation.Prerender:mode/no_state_prefetch

We can probably have a helper function to do this if needed.

[droger, 2016/12/05 14:48:27]

Done.

[pasko, 2016/12/08 15:23:44]

In the last version of the code, the above instructions would not work, or would
they? I just could not find where it is documented in the end. Let's put the
howto on enabling/disabling the feature from command-line in the design doc? In
the code would also be OK, but needs upload/review/commit overhead.

[droger, 2016/12/08 15:47:30]

I think they would work, but I did not need it in this CL.
Instead I called directly prerender::PrerenderManager::SetMode() from the test
setup.

This command line is still needed to set the prerender mode from the command
line.
Another option is to use about://flags.

I'll document this.

-                                    switches::kPrerenderModeSwitchValueEnabled);
   }
 
   static void Prerender_Arguments(
       const std::string& extension_id,
       uint16_t port,
       const base::Closure& quit_when_idle_closure,
       std::unique_ptr<std::vector<scoped_refptr<Action>>> i) {
     quit_when_idle_closure.Run();
 
     ASSERT_TRUE(i->size());
@@ skipping 65 matching lines @@
   activity_log->GetFilteredActions(
       ext->id(), Action::ACTION_ANY, "", "", "", -1,
       base::Bind(ActivityLogPrerenderTest::Prerender_Arguments, ext->id(), port,
                  run_loop.QuitWhenIdleClosure()));
 
   // Allow invocation of Prerender_Arguments
   run_loop.Run();
 }
 
 }  // namespace extensions