Implement popup autoresizing for Safari

By default, Safari doesn't resize the popup to its content, but it's
possible to set the size pragmatically.
The popup will be resized every time when a change happens in the DOM tree.

src/js/vapi-common.js

@@ -49,12 +49,35 @@ if (window.chrome) {
 
     // update popover size to its content
     if (safari.self.identifier === 'popover' && safari.self) {
-        vAPI.updatePopoverSize = function() {
-            safari.self.width = document.body.clientWidth;
-            safari.self.height = document.body.clientHeight;
-        };
+        (function() {
+            var upadteTimer = null;
+            var resizePopover = function() {
+                if (upadteTimer) {
+                    return;
+                }
 
-        setTimeout(vAPI.updatePopoverSize, 200);
+                upadteTimer = setTimeout(function() {
+                    safari.self.width = document.body.clientWidth;
+                    safari.self.height = document.body.clientHeight;
+                    upadteTimer = null;
+                }, 50);
+            };
+
+            var mutObs = window.MutationObserver || window.WebkitMutationObserver;
+
+            if (mutObs) {
+                (new mutObs(resizePopover)).observe(document, {
+                    childList: true,
+                    attributes: true,
+                    characterData: true,
+                    subtree: true
+                });
+            }
+            else {
+                // Safari doesn't support DOMAttrModified
+                document.addEventListener('DOMSubtreeModified', resizePopover);
+            }
+        })();
     }
 }