bug 1249162 - Fix unwanted thumb shifts when starting APZ drag r=botond

Fix thumb position determination in these places:
- nsSliderFrame::StartAPZDrag() -- Constrain sliderTrack using
  GetXULClientRect() to match dimensions used in
  SetCurrentThumbPosition() and DoXULLayout(). This is what caused
  the scaling/offset mismatch.
- nsSliderFrame::StartAPZDrag() -- Adjust nonsensical calculation of
  cssSliderTrack. Should be sliderFrame + scrollbarFramePosition -
  compositionBoundsPosition, to get coordinates relative to the same
  region as that of APZ event coordinates.
- AsyncDragMetrics -- Make mScrollTrack and mScrollbarDragOffset float
  instead of int coordinates.
- AsyncPanZoomController::HandleDragEvent() -- Use
  GetAxisLength(scrollTrack) instead of GetAxisEnd(scrollTrack) in
  calculation of scrollMax.
- AsyncPanZoomController::HandleDragEvent() -- Only change position on
  MOUSE_MOVE.

Additional refactors:
- Rename HitTestingTreeNode::GetScrollSize() to GetScrollThumbLength(),
  along with related functions/variables.
- Rename AsyncPanZoomController::GetAxisSize() to GetAxisLength().
- Rename cf to scrollFrame in nsSliderFrame::StartAPZDrag().

MozReview-Commit-ID: CIsU8Pj6qfa

gfx/layers/LayerMetricsWrapper.h

@@ -423,7 +423,7 @@ public:
     return mLayer->GetScrollbarTargetContainerId();
   }
 
-  int32_t GetScrollbarSize() const
+  int32_t GetScrollThumbLength() const
   {
     if (GetScrollbarDirection() == Layer::VERTICAL) {
       return mLayer->GetVisibleRegion().GetBounds().height;