Add FirstScrollLatencyMeasurement flag

This CL adds a chrome://flag for controlling whether or not we run code
to track First Scroll Latency. This lets users and release management
processes more easily disable this feature if it introduces stability or
performance issues.

Bug: 922980
Change-Id: I9a941890f8b5f5b26187b2b850f916e38eb325ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135869
Reviewed-by: Kinuko Yasuda <[email protected]>
Commit-Queue: Tom McKee <[email protected]>
Cr-Commit-Position: refs/heads/master@{#756649}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 61edae1..4cd528b 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -539,6 +539,9 @@
 extern const char kFilteringScrollPredictionName[];
 extern const char kFilteringScrollPredictionDescription[];
 
+extern const char kFirstScrollLatencyMeasurementName[];
+extern const char kFirstScrollLatencyMeasurementDescription[];
+
 extern const char kFractionalScrollOffsetsName[];
 extern const char kFractionalScrollOffsetsDescription[];