[iOS] Modify comments and default value for app relaunch EG2 utilities.

- Modified comments for |-ensureAppLaunchedWithConfiguration:| method of
  |AppLaunchManager| reminding users to choose suitable place to call
  this method for better efficiency.
- Changed default value of |RelaunchPolicy| in |AppLaunchConfiguration|
  to |NoForceRelaunchAndResetState| which is more commonly used when
  user relaunches the app for different enabled features.

Bug: 1009017
Change-Id: I523304d387d5f873a33cc0b72023d9eafe6e7c20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2073137Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744820}

ios/testing/earl_grey/app_launch_configuration.h

@@ -39,7 +39,7 @@ struct AppLaunchConfiguration {
   // Enabled trigger variations.
   std::vector<variations::VariationID> trigger_variations_enabled;
   // Relaunch policy.
-  RelaunchPolicy relaunch_policy = NoForceRelaunchAndKeepState;
+  RelaunchPolicy relaunch_policy = NoForceRelaunchAndResetState;
 };
 
 #endif  // IOS_TESTING_EARL_GREY_APP_LAUNCH_CONFIGURATION_H_

ios/testing/earl_grey/app_launch_manager.h

@@ -40,12 +40,13 @@
 // Otherwise, the app will be activated instead of (re)launched.
 // Will wait until app is activated or launched, and fail the test if it
 // fails to do so.
-// |relaunchPolicy| controls relaunch manners.
+// |configuration| sets features, variations and relaunch manners.
+// If you're trying to call this method in |-setUp()|, please put it in
+// |-launchAppForTestMethod()| instead for better efficiency.
 - (void)ensureAppLaunchedWithConfiguration:
     (AppLaunchConfiguration)configuration;
 
-// See ensureAppLaunchedWithFeaturesEnabled:disabled:variations:
-// triggerVariations:relaunchPolicy: for details.
+// See ensureAppLaunchedWithConfiguration: for details.
 - (void)ensureAppLaunchedWithFeaturesEnabled:
             (std::vector<base::Feature>)featuresEnabled
                                     disabled:(std::vector<base::Feature>)