Fix subtle bugs in sky-scrollable

Previously the scrollbar wasn't constrained to the scrollable element and we
had messed up the math on the fling curve.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/947303003

sky/framework/fling-curve.dart

diff --git a/sky/framework/fling-curve.dart b/sky/framework/fling-curve.dart
index 6a476d7..3a9f54f 100644
--- a/sky/framework/fling-curve.dart
+++ b/sky/framework/fling-curve.dart
@@ -36,7 +36,7 @@
 
   FlingCurve(double velocity, double startTime) {
     double startingVelocity = math.min(_kMaxVelocity, velocity.abs());
-    _timeOffset = _velocityAtTime(startingVelocity);
+    _timeOffset = _timeAtVelocity(startingVelocity);
     _positionOffset = _positionAtTime(_timeOffset);
     _startTime = startTime / 1000.0;
     _previousPosition = 0.0;

sky/framework/sky-drawer.sky

diff --git a/sky/framework/sky-drawer.sky b/sky/framework/sky-drawer.sky
index 9a54caa..906f4f4 100644
--- a/sky/framework/sky-drawer.sky
+++ b/sky/framework/sky-drawer.sky
@@ -19,7 +19,7 @@
       right: 0;
     }
     #content {
-      background-color: yellow;
+      background-color: white;
       will-change: transform;
       position: absolute;
       width: 256px;

sky/framework/sky-scrollable.sky

diff --git a/sky/framework/sky-scrollable.sky b/sky/framework/sky-scrollable.sky
index fa287a0..3e41d88 100644
--- a/sky/framework/sky-scrollable.sky
+++ b/sky/framework/sky-scrollable.sky
@@ -11,6 +11,7 @@
   :host {
     overflow: hidden;
     position: relative;
+    will-change: transform;
   }
   #scrollable {
     will-change: transform;

sky/viewer/converters/input_event_types.cc

diff --git a/sky/viewer/converters/input_event_types.cc b/sky/viewer/converters/input_event_types.cc
index d5bb31e..3aa297d 100644
--- a/sky/viewer/converters/input_event_types.cc
+++ b/sky/viewer/converters/input_event_types.cc
@@ -108,10 +108,10 @@
       break;
     case mojo::EVENT_TYPE_SCROLL_FLING_START:
       web_event->type = blink::WebInputEvent::GestureFlingStart;
-      // TODO(abarth): Why don't we need to divide by the device_pixel_ratio
-      // here? For some reason, this seems to get the right velocity.
-      web_event->data.flingStart.velocityX = event->gesture_data->velocity_x;
-      web_event->data.flingStart.velocityY = event->gesture_data->velocity_y;
+      web_event->data.flingStart.velocityX =
+          event->gesture_data->velocity_x / device_pixel_ratio;
+      web_event->data.flingStart.velocityY =
+          event->gesture_data->velocity_y / device_pixel_ratio;
       break;
     case mojo::EVENT_TYPE_SCROLL_FLING_CANCEL:
       web_event->type = blink::WebInputEvent::GestureFlingCancel;