Bug 898877 - Prevent pages from getting stuck without the dynamic toolbar. r=Cwiiis

The problematic scenario is when the page is exactly the height of the screen
(with dynamic toolbar not visible). In this case, the scrollable() function in
Axis.java returns false on the vertical axis, and so the JavaPanZoomController
never does any scrolling. This in turns means that the scrollBy code in
LayerMarginsAnimator never gets to run, so you can never drag the toolbar back
into being visible. The patch ensures that scrollable() returns true when some
or all of the margins are not visible, ensuring that in these scenarios the
user can still scroll the toolbar back onto the screen. This patch also adds
some comments/asserts to verify the new code is threadsafe.

mobile/android/base/gfx/JavaPanZoomController.java

@@ -1056,6 +1056,12 @@ class JavaPanZoomController
         protected float getPageStart() { return getMetrics().pageRectLeft; }
         @Override
         protected float getPageLength() { return getMetrics().getPageWidthWithMargins(); }
+        @Override
+        protected boolean marginsHidden() {
+            ImmutableViewportMetrics metrics = getMetrics();
+            RectF maxMargins = mTarget.getMaxMargins();
+            return (metrics.marginLeft < maxMargins.left || metrics.marginRight < maxMargins.right);
+        }
     }
 
     private class AxisY extends Axis {
@@ -1068,6 +1074,12 @@ class JavaPanZoomController
         protected float getPageStart() { return getMetrics().pageRectTop; }
         @Override
         protected float getPageLength() { return getMetrics().getPageHeightWithMargins(); }
+        @Override
+        protected boolean marginsHidden() {
+            ImmutableViewportMetrics metrics = getMetrics();
+            RectF maxMargins = mTarget.getMaxMargins();
+            return (metrics.marginTop < maxMargins.top || metrics.marginBottom < maxMargins.bottom);
+        }
     }
 
     /*