Bug 1161392 - Enlarge touch area for AccessibleCaret. r=roc

Porting the patch for Touch/SelectionCarets in bug 1021499 to
AccessibleCaret.

layout/base/AccessibleCaret.cpp

@@ -254,9 +254,10 @@ AccessibleCaret::SetCaretElementStyle(const nsRect& aRect)
 {
   nsPoint position = CaretElementPosition(aRect);
   nsAutoString styleStr;
-  styleStr.AppendPrintf("left: %dpx; top: %dpx;",
+  styleStr.AppendPrintf("left: %dpx; top: %dpx; padding-top: %dpx;",
                         nsPresContext::AppUnitsToIntCSSPixels(position.x),
-                        nsPresContext::AppUnitsToIntCSSPixels(position.y));
+                        nsPresContext::AppUnitsToIntCSSPixels(position.y),
+                        nsPresContext::AppUnitsToIntCSSPixels(aRect.height));
 
   float zoomLevel = GetZoomLevel();
   styleStr.AppendPrintf(" width: %.2fpx; height: %.2fpx; margin-left: %.2fpx",

layout/base/AccessibleCaret.h

@@ -165,11 +165,11 @@ private:
   // Remove caret element from custom content container.
   void RemoveCaretElement(nsIDocument* aDocument);
 
-  // The bottom-center of the imaginary caret to which this AccessibleCaret is
+  // The top-center of the imaginary caret to which this AccessibleCaret is
   // attached.
   static nsPoint CaretElementPosition(const nsRect& aRect)
   {
-    return aRect.TopLeft() + nsPoint(aRect.width / 2, aRect.height);
+    return aRect.TopLeft() + nsPoint(aRect.width / 2, 0);
   }
 
   class DummyTouchListener final : public nsIDOMEventListener

layout/style/ua.css

@@ -322,8 +322,10 @@ div:-moz-native-anonymous.moz-accessiblecaret > div.bar {
 }
 
 div:-moz-native-anonymous.moz-accessiblecaret > div.image {
-  background-position: center center;
-  background-size: 100% 100%;
+  background-position: center bottom;
+  background-size: 100%;
+  background-repeat: no-repeat;
+  bottom: 0;
   width: 100%;
   height: 100%;