Add feature flag: Navigation response using a Mojo DataPipe.

* Add content::feature NavigationMojoResponse
* Add chrome://flags/#navigation-mojo-response

See the others CLs in this series:
[1/3] this CL.
[2/3] https://chromium-review.googlesource.com/c/741237
[3/3] https://chromium-review.googlesource.com/c/753738

Design doc: https://goo.gl/Rrrc7n

Bug: 705744
Change-Id: I8763b7d711d1ac0d06aad56130a009e3ee8a75d0
Reviewed-on: https://chromium-review.googlesource.com/739502
Reviewed-by: Camille Lamy <[email protected]>
Reviewed-by: Nasko Oskov <[email protected]>
Commit-Queue: Arthur Sonzogni <[email protected]>
Cr-Commit-Position: refs/heads/master@{#514794}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 8163513..920d0fc 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -75,6 +75,9 @@
 extern const char kBrowserSideNavigationName[];
 extern const char kBrowserSideNavigationDescription[];
 
+extern const char kNavigationMojoResponseName[];
+extern const char kNavigationMojoResponseDescription[];
+
 extern const char kBrowserTaskSchedulerName[];
 extern const char kBrowserTaskSchedulerDescription[];