Fix paper tooltip fit to visible bounds.

There are 3 cases that can happen with fit to visible bounds (Using left/right for simplicity here)

1) Off the right of the screen
2) Off the left of the screen
3) In the middle of the screen (no adjustment required)

Paper tooltip failed to handle case (3).

Fix that.

paper-tooltip.js

@@ -473,17 +473,21 @@ Polymer({
       if (parentRect.left + tooltipLeft + thisRect.width > window.innerWidth) {
         this.style.right = '0px';
         this.style.left = 'auto';
-      } else {
+      } else if (parentRect.left + tooltipLeft < 0) {
         this.style.left = Math.max(0, tooltipLeft) + 'px';
         this.style.right = 'auto';
+      } else {
+        this.style.left = tooltipLeft + 'px';
       }
       // Clip the top/bottom side.
       if (parentRect.top + tooltipTop + thisRect.height > window.innerHeight) {
         this.style.bottom = (parentRect.height - targetTop + offset) + 'px';
         this.style.top = 'auto';
-      } else {
+      } else if (parentRect.top + tooltipTop < 0) {
         this.style.top = Math.max(-parentRect.top, tooltipTop) + 'px';
         this.style.bottom = 'auto';
+      } else {
+        this.style.top = tooltipTop + 'px';
       }
     } else {
       this.style.left = tooltipLeft + 'px';