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

chrome/browser/extensions/activity_log/activity_log_browsertest.cc

Index: chrome/browser/extensions/activity_log/activity_log_browsertest.cc
diff --git a/chrome/browser/extensions/activity_log/activity_log_browsertest.cc b/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
index e9248b0041ba9cea9bb867bb3c4ee2b11871a215..b290d7a05d0d8633b078ce2560d7da37455f8bd0 100644
--- a/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
+++ b/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
@@ -36,8 +36,6 @@ class ActivityLogPrerenderTest : public ExtensionApiTest {
   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(