[chromecast] Notify applications about ongoing back gesture.

Adds a new method, "GestureProgress" to CastContentWindow::Delegate's
interface which can be used to pass on to the application the
knowledge of an ongoing system gesture.

Bug: internal b/109888887
Test: manual
Change-Id: I4db39537274e16bcb35f84013cc01a71e7c14f3e
Reviewed-on: https://chromium-review.googlesource.com/1093025Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565774}

chromecast/browser/cast_back_gesture_dispatcher.cc

@@ -39,8 +39,12 @@ void CastBackGestureDispatcher::HandleSideSwipeBegin(
 void CastBackGestureDispatcher::HandleSideSwipeContinue(
     CastSideSwipeOrigin swipe_origin,
     const gfx::Point& touch_location) {
-  if (!dispatched_back_ && swipe_origin == CastSideSwipeOrigin::LEFT &&
-      touch_location.x() >= horizontal_threshold_) {
+  if (swipe_origin != CastSideSwipeOrigin::LEFT) {
+    return;
+  }
+
+  delegate_->GestureProgress(GestureType::GO_BACK, touch_location);
+  if (!dispatched_back_ && touch_location.x() >= horizontal_threshold_) {
     dispatched_back_ = true;
     delegate_->ConsumeGesture(GestureType::GO_BACK);
   }

chromecast/browser/cast_content_window.h

@@ -83,8 +83,12 @@ class CastContentWindow {
     // called prior to ConsumeGesture().
     virtual bool CanHandleGesture(GestureType gesture_type) = 0;
 
-    // Consume and handle a UI gesture. Returns whether the gesture was
-    // handled or not.
+    // Called while a system UI gesture is in progress.
+    virtual void GestureProgress(GestureType gesture_type,
+                                 const gfx::Point& touch_location){};
+
+    // Consume and handle a completed UI gesture. Returns whether the gesture
+    // was handled or not.
     virtual bool ConsumeGesture(GestureType gesture_type) = 0;
 
     // Notify visibility change for this window.