Long Press Back Button to Show History

- Show navigation history popup (align bottom and center) when long pressing
Android system back button.
- Add *Show full history* item, allow users go to history page.
- Add flag: LongPressBackForHistory for this feature, default disabled.

Bug: 800033
Change-Id: Ie3d755a7c9795235b40cf3c0ed6ffaf460dc3f21
Reviewed-on: https://chromium-review.googlesource.com/1144641
Commit-Queue: Charlie Ma <[email protected]>
Reviewed-by: Theresa <[email protected]>
Reviewed-by: Ted Choc <[email protected]>
Cr-Commit-Position: refs/heads/master@{#578726}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 7abe3d8..a7c1e67 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1469,6 +1469,9 @@
 extern const char kLanguagesPreferenceName[];
 extern const char kLanguagesPreferenceDescription[];
 
+extern const char kLongPressBackForHistoryName[];
+extern const char kLongPressBackForHistoryDescription[];
+
 extern const char kLsdPermissionPromptName[];
 extern const char kLsdPermissionPromptDescription[];