scheduler: Add main thread gesture as a detected use case

components/scheduler/renderer/renderer_scheduler.h

Index: components/scheduler/renderer/renderer_scheduler.h
diff --git a/components/scheduler/renderer/renderer_scheduler.h b/components/scheduler/renderer/renderer_scheduler.h
index 51ed4b18f2c1003774f1e4398762285b52c1f02e..c6116ea79bdaa61abb4c5e48d3d6257ec6c66103 100644
--- a/components/scheduler/renderer/renderer_scheduler.h
+++ b/components/scheduler/renderer/renderer_scheduler.h
@@ -45,12 +45,29 @@ class SCHEDULER_EXPORT RendererScheduler : public ChildScheduler {
 
   // Keep RendererScheduler::UseCaseToString in sync with this enum.
   enum class UseCase {
+    // No active use case detected.
     NONE,
+    // A continuous gesture (e.g., scroll, pinch) which is being driven by the
+    // compositor thread.
     COMPOSITOR_GESTURE,
-    MAIN_THREAD_GESTURE,
-    SYNCHRONIZED_GESTURE,  // Both threads in the critical path.
+    // An unspecified touch gesture which is being handled by the main thread.
+    // Note that since we don't have a full view of the use case, we should be
+    // careful to prioritize all work equally.
+    MAIN_THREAD_CUSTOM_INPUT_HANDLING,
+    // A continuous gesture (e.g., scroll, pinch) which is being driven by the
+    // compositor thread but also observed by the main thread. An example is
+    // synchronized scrolling where a scroll listener on the main thread changes
+    // page layout based on the current scroll position.
+    SYNCHRONIZED_GESTURE,
+    // A gesture has recently started and we are about to run main thread touch
+    // listeners to find out the actual gesture type. To minimize touch latency,
+    // only input handling work should run in this state.
     TOUCHSTART,
+    // The page is loading.
     LOADING,
+    // A continuous gesture (e.g., scroll) which is being handled by the main
+    // thread.
+    MAIN_THREAD_GESTURE,
     // Must be the last entry.
     USE_CASE_COUNT,
     FIRST_USE_CASE = NONE,