Fix import of multithreaded Chrome profiles (#194)

In #160, I wrote code which incorrectly assumed that at most one profile would be active at a time. It turns out this assumption is incorrect because of webworkers! This PR introduces a fix which correctly separates samples taken on the main thread from samples taken on worker threads, and allows viewing both in speedscope.

Fixes #171

sample/programs/javascript/worker.html

@@ -0,0 +1,28 @@
+<script>
+  function banana() {
+    let prod = 1
+    for (let i = 1; i < 1000; i++) {
+      prod *= i
+    }
+    return prod
+  }
+
+  function apple() {
+    for (let i = 0; i < 1000; i++) {
+      banana()
+    }
+  }
+
+  let bounces = 0
+
+  const worker = new Worker('./worker.js')
+
+  worker.onmessage = function() {
+    apple()
+    if (bounces++ < 10) {
+      worker.postMessage('ping')
+    }
+  }
+
+  worker.postMessage('ping')
+</script>

sample/programs/javascript/worker.js

@@ -0,0 +1,18 @@
+function gamma() {
+  let prod = 1
+  for (let i = 1; i < 1000; i++) {
+    prod *= i
+  }
+  return prod
+}
+
+function alpha() {
+  for (let i = 0; i < 1000; i++) {
+    gamma()
+  }
+}
+
+onmessage = function(e) {
+  alpha()
+  postMessage('pong')
+}