[Home Page Button] Adds flags & metrics

> Creates flags for home page button
> Logs preference changes in new histogram
> Sets home page default as NTP if home page was displayed by flag

Bug: 822469
Change-Id: I2c0a00f11de254f663e453f32aaab7d43ad7e761
Reviewed-on: https://chromium-review.googlesource.com/972461
Commit-Queue: Theresa <[email protected]>
Reviewed-by: Ted Choc <[email protected]>
Reviewed-by: Theresa <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Mark Pearson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#546162}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index db709be..c236eb4 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1243,6 +1243,9 @@
 extern const char kGrantNotificationsToDSEName[];
 extern const char kGrantNotificationsToDSENameDescription[];
 
+extern const char kHomePageButtonName[];
+extern const char kHomePageButtonDescription[];
+
 extern const char kInterestFeedContentSuggestionsName[];
 extern const char kInterestFeedContentSuggestionsDescription[];