[TabModal] Enable tab modal javascript dialog by default

Bug: 687010
Change-Id: I304d68c8ade4e6cddb2017881b80b9300d0e512d
Reviewed-on: https://chromium-review.googlesource.com/1170188Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582191}

chrome/browser/android/chrome_feature_list.cc

@@ -401,7 +401,7 @@ const base::Feature kSpecialLocaleWrapper{"SpecialLocaleWrapper",
                                           base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kTabModalJsDialog{"TabModalJsDialog",
-                                      base::FEATURE_DISABLED_BY_DEFAULT};
+                                      base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kTabReparenting{"TabReparenting",
                                     base::FEATURE_ENABLED_BY_DEFAULT};