[Signin][Android] Enable personalized signin promos by default

This CL switches the default state of AndroidSigninPromos feature to
FEATURE_ENABLED_BY_DEFAULT.

Bug: 792430
Change-Id: I09e7c26db6b899c593f69b9d459bbe03171a115b
Reviewed-on: https://chromium-review.googlesource.com/810744Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522390}

chrome/browser/android/chrome_feature_list.cc

@@ -167,7 +167,7 @@ const base::Feature kAndroidPaymentApps{"AndroidPaymentApps",
                                         base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kAndroidSigninPromos{"AndroidSigninPromos",
-                                         base::FEATURE_DISABLED_BY_DEFAULT};
+                                         base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kCCTBackgroundTab{"CCTBackgroundTab",
                                       base::FEATURE_ENABLED_BY_DEFAULT};