Make OffscreenCanvas histogram thread safe

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-multiple-worker-commit.html

+<!DOCTYPE>
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script id="myWorker" type="text/worker">
+self.onmessage = function(e) {
+    var offscreenCanvas = e.data;
+    var offscreen2d = offscreenCanvas.getContext("2d");
+    offscreen2d.commit();
+    self.postMessage("worker done");
+};
+</script>
+<script>
+function makeWorker(script) {
+   var blob = new Blob([script]);
+   return new Worker(URL.createObjectURL(blob));
+}
+
+// Regression test for crbug.com/663916
+async_test(function(t) {
+    var canvas1 = document.createElement('canvas');
+    canvas1.width = canvas1.height = 10;
+    var offscreenCanvas1 = canvas1.transferControlToOffscreen();
+
+    var canvas2 = document.createElement('canvas');
+    canvas2.width = canvas2.height = 10;
+    var offscreenCanvas2 = canvas2.transferControlToOffscreen();
+
+    var worker1 = makeWorker(document.getElementById("myWorker").textContent);

[Justin Novosad, 2016/11/15 20:24:58]

Nit: no need for an argument for makeWorker because it always loads the same
script.

+    var worker2 = makeWorker(document.getElementById("myWorker").textContent);
+    var returnedWorkerCounter = 0;
+    worker1.onmessage = function (e) {
+        returnedWorkerCounter++;
+        if (returnedWorkerCounter == 2)
+            t.done();
+    };
+    worker2.onmessage = function (e) {
+        returnedWorkerCounter++;
+        if (returnedWorkerCounter == 2)
+            t.done();
+    };
+    worker1.postMessage(offscreenCanvas1, [offscreenCanvas1]);
+    worker2.postMessage(offscreenCanvas2, [offscreenCanvas2]);
+}, "Calling OffscreenCanvas.commit() from two workers should not crash.");
+
+</script>