WIP - compositor worker mega patch.

third_party/WebKit/ManualTests/compositor-worker/sticky/js/viewport-observer.js

+// Incomplete / Naive implementation of ViewportObserver based on spec at:
+// https://docs.google.com/document/d/14y3ReQo_TU8N81V1okwmfT5LtRxj-20UiNJ3ddU9vsI/edit?pli=1#heading=h.epew5hgbndxp
+window.ViewportObserver = function() {
+
+  function ViewportObserver(callback) {
+    this.callback_ = callback;
+    this.targets_ = [];
+    this.changedRecords_ = [];
+    this.notifyNow_ = this.notify_.bind(this);
+  }
+
+  ViewportObserver.prototype.observe = function(target) {
+    if (this.targets_.indexOf(target) != -1)
+      return;
+    var listenOn = target == document.scrollingElement ? document : target;
+    listenOn.addEventListener('scroll', this.onScroll_.bind(this, target));
+    this.targets_.push(target);
+    this.onScroll_(target);
+  };
+
+  ViewportObserver.prototype.onScroll_ = function(target) {
+    this.changedRecords_.push({
+      time: performance.now(),
+      viewport: getViewportRect(target),
+      element: target
+    });
+    this.notify_();
+  };
+
+  ViewportObserver.prototype.scheduleNotify_ = function() {
+    if (!this.timeoutId_)
+      this.timeoutId_ = setTimeout(this.notifyNow_, 0);
+  };
+
+  ViewportObserver.prototype.notify_ = function() {
+    clearTimeout(this.timeoutId_);
+    this.timeoutId_ = undefined;
+    this.callback_(this.changedRecords_);
+    this.changedRecords_.length = 0;
+  };
+
+  return ViewportObserver;
+}();