Preserve alpha channel of 1.0 when emulating RGB back buffer.

content/test/data/gpu/pixel_webgl2_util.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// READ BEFORE UPDATING:
+// If this file is updated, make sure to increment the "revision" value of any
+// tests that use this file in content/test/gpu/page_sets/pixel_tests.py. This
+// will ensure that the baseline images are regenerated on the next run.
+
+var g_swapsBeforeAckUtil = 15;
+
+// TODO(kbr): We should test premultiplyAlpha as well.
+function initGL(canvas, antialias, alpha)
+{
+  var gl = null;
+  try {
+    gl = canvas.getContext("webgl2",
+                           {"alpha": alpha, "antialias":antialias});
+  } catch (e) {}
+  return gl;
+}
+
+function waitAndSignalHarness()
+{
+  if (g_swapsBeforeAckUtil == 0) {
+    if (window.domAutomationController) {
+      domAutomationController.setAutomationId(1);
+      domAutomationController.send("SUCCESS");
+    }
+  } else {
+    g_swapsBeforeAckUtil--;
+    requestAnimationFrame(waitAndSignalHarness);
+  }
+}