Add flag for Impulse scroll animations

This CL adds a feature flag for Impulse-style scroll animations. Note
that this also adds cc/base/features.*

Bug: 1030940
Change-Id: I28eefd38417be5dbcd5fec28c20cf9697b13d061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948536
Commit-Queue: Matt Amert <[email protected]>
Reviewed-by: Eric Karl <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Khushal <[email protected]>
Cr-Commit-Position: refs/heads/master@{#722838}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 03fa7f0..871a0ab 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -735,6 +735,9 @@
 extern const char kCompositorThreadedScrollbarScrollingName[];
 extern const char kCompositorThreadedScrollbarScrollingDescription[];
 
+extern const char kImpulseScrollAnimationsName[];
+extern const char kImpulseScrollAnimationsDescription[];
+
 extern const char kInProductHelpDemoModeChoiceName[];
 extern const char kInProductHelpDemoModeChoiceDescription[];