Add feature flag for scrollbar scroll injecting gestures

Given that there were a couple regressions the first time I changed this
behavior for scrollbar presses (see crrev.com/1580588 and the revert,
crrev.com/1616341), I'm introducing a feature flag so that we can
have a kill switch.

R=bokan@chromium.org, dcheng@chromium.org, dtapuska@chromium.org

Bug: 954007
Change-Id: I635c5f39a798986c820287c47474f8b80a473307
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617140
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662075}

third_party/blink/common/features.cc

@@ -271,6 +271,10 @@ const base::Feature kBlockingFocusWithoutUserActivation{
 const base::Feature kAudioWorkletRealtimeThread{
     "AudioWorkletRealtimeThread", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Use scroll gestures for scrollbar scrolls (see https://crbug.com/954007).
+const base::Feature kScrollbarInjectScrollGestures{
+    "ScrollbarInjectScrollGestures", base::FEATURE_DISABLED_BY_DEFAULT};
+
 bool IsOffMainThreadSharedWorkerScriptFetchEnabled() {
   // Off-the-main-thread shared worker script fetch depends on PlzSharedWorker
   // (NetworkService).

third_party/blink/public/common/features.h

@@ -82,6 +82,7 @@ BLINK_COMMON_EXPORT extern const base::Feature
     kWebFontsCacheAwareTimeoutAdaption;
 BLINK_COMMON_EXPORT extern const base::Feature
     kBlockingFocusWithoutUserActivation;
+BLINK_COMMON_EXPORT extern const base::Feature kScrollbarInjectScrollGestures;
 
 BLINK_COMMON_EXPORT extern const base::Feature kAudioWorkletRealtimeThread;