Add a feature flag for Harmony/Views Cast dialog

Add the ViewsCastDialog feature, which replaces the WebUI Cast dialog
with a Views toolkit dialog when enabled. The feature flag is also made
accessible through chrome://flags.

The implementation for the feature will land in the future.

Bug: 825989
Change-Id: Ib69a2a56362093d0e42e53b747ba0fdc9f0c5778
Reviewed-on: https://chromium-review.googlesource.com/982635
Commit-Queue: Takumi Fujimoto <[email protected]>
Reviewed-by: mark a. foltz <[email protected]>
Cr-Commit-Position: refs/heads/master@{#547385}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 61db0f5..88e026e 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -498,6 +498,9 @@
 extern const char kHarfbuzzRendertextName[];
 extern const char kHarfbuzzRendertextDescription[];
 
+extern const char kViewsCastDialogName[];
+extern const char kViewsCastDialogDescription[];
+
 extern const char kHideNonActiveAppsFromShelfName[];
 extern const char kHideNonActiveAppsFromShelfDescription[];