Allow disabling StartupTabPreloader from the Intent.

The AGA experiment id changes in:
  https://chromium-review.googlesource.com/c/chromium/src/+/2338934
does not work when the StartupTabPreloader is used. Since the
StartupTabPreloader launches the navigation before all of native is
loaded, fixing this isn't simply a matter of moving the experiment
reading code earlier - it would involve changing Chrome's startup logic.

This change allows clients to disable the StartupTabPreloader to ensure
that the experiment logic in the linked CL is triggered (at the cost of
a longer time to navigation on cold start).

Change-Id: Ie83b4f38705084c567d7c20821600de7cdf15695
Bug: 1111941
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356505
Commit-Queue: Peter Conn <peconn@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Auto-Submit: Peter Conn <peconn@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798203}

chrome/android/java/src/org/chromium/chrome/browser/init/StartupTabPreloader.java

@@ -42,6 +42,9 @@ import org.chromium.url.GURL;
  * is created. This is done to improve startup latency.
  */
 public class StartupTabPreloader implements ProfileManager.Observer, Destroyable {
+    private static final String EXTRA_DISABLE_STARTUP_TAB_PRELOADER =
+            "org.chromium.chrome.browser.init.DISABLE_STARTUP_TAB_PRELOADER";
+
     private final Supplier<Intent> mIntentSupplier;
     private final ActivityLifecycleDispatcher mActivityLifecycleDispatcher;
     private final WindowAndroid mWindowAndroid;
@@ -154,6 +157,9 @@ public class StartupTabPreloader implements ProfileManager.Observer, Destroyable
         if (mTab != null) return false;
 
         Intent intent = mIntentSupplier.get();
+        if (IntentUtils.safeGetBooleanExtra(intent, EXTRA_DISABLE_STARTUP_TAB_PRELOADER, false)) {
+            return false;
+        }
         if (mIntentHandler.shouldIgnoreIntent(intent)) return false;
         if (getUrlFromIntent(intent) == null) return false;