Add test coverage for --prerender=disabled switch

This adds a test to ensure that the disable switch is respected
by ConfigurePrerender in prerender_field_trial.cc.

BUG=455449

Review URL: https://codereview.chromium.org/897983003

Cr-Commit-Position: refs/heads/master@{#315336}

AUTHORS

@@ -117,6 +117,7 @@ David Erceg <erceg.david@gmail.com>
 David Futcher <david.mike.futcher@gmail.com>
 David Leen <davileen@amazon.com>
 David McAllister <mcdavid@amazon.com>
+David Spellman <dspell@amazon.com>
 Deepak Dilip Borade <deepak.db@samsung.com>
 Deepak Mittal <deepak.m1@samsung.com>
 Deepak Singla <deepak.sa@samsung.com>

chrome/browser/prerender/prerender_unittest.cc

@@ -15,6 +15,7 @@
 #include "base/time/time.h"
 #include "chrome/browser/net/prediction_options.h"
 #include "chrome/browser/prerender/prerender_contents.h"
+#include "chrome/browser/prerender/prerender_field_trial.h"
 #include "chrome/browser/prerender/prerender_handle.h"
 #include "chrome/browser/prerender/prerender_link_manager.h"
 #include "chrome/browser/prerender/prerender_manager.h"
@@ -405,6 +406,16 @@ class PrerenderTest : public testing::Test {
   base::FieldTrialList field_trial_list_;
 };
 
+TEST_F(PrerenderTest, PrerenderRespectsDisableFlag) {
+  ASSERT_TRUE(PrerenderManager::IsPrerenderingPossible());
+  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
+  command_line->AppendSwitchASCII(
+    switches::kPrerenderMode,
+    switches::kPrerenderModeSwitchValueDisabled);
+  prerender::ConfigurePrerender(*command_line);
+  ASSERT_FALSE(PrerenderManager::IsPrerenderingPossible());
+}
+
 TEST_F(PrerenderTest, FoundTest) {
   GURL url("http://www.google.com/");
   DummyPrerenderContents* prerender_contents =