Extend canvas getContextAttributes WPTest

This CL extend the WPTest for Canvas getContextAttributes to include
the |desynchronized|.

This CL covers the flag WPT test extension mentioned in
whatwg/html#4360 (which addressed
whatwg/html#4087).

Bug: 944199, 945835
Change-Id: Ibb339c66014d7579023f75c7f2cef99e3f03d351
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538491
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644332}

2dcontext/context-attributes/getContextAttributes.html

@@ -6,22 +6,31 @@
 var testScenarios = [
     {testDescription: "Test default context creation attributes",
         canvasContextAttributes: {},
-        expectedContextAttributes: {alpha : true}},
+        expectedContextAttributes: {alpha : true, desynchronized: false}},
     {testDescription: "Test context creation attributes alpha: true",
         canvasContextAttributes: {alpha: true},
         expectedContextAttributes: {alpha : true}},
     {testDescription: "Test context creation attributes alpha: false",
         canvasContextAttributes: {alpha: false},
         expectedContextAttributes: {alpha : false}},
+    {testDescription: "Test context creation attributes desynchronized: false",
+        canvasContextAttributes: {desynchronized: false},
+        expectedContextAttributes: {desynchronized : false}},
 ];
 
 function runTestScenario(testScenario) {
     var t = test(function() {
         var canvas = document. createElement('canvas');
         var ctx = canvas.getContext('2d', testScenario.canvasContextAttributes);
         var contextAttributes = ctx.getContextAttributes();
-        assert_equals(contextAttributes.alpha,
-            testScenario.expectedContextAttributes.alpha);
+        if (testScenario.expectedContextAttributes.alpha !== undefined) {
+            assert_equals(contextAttributes.alpha,
+                testScenario.expectedContextAttributes.alpha);
+        }
+        if (testScenario.expectedContextAttributes.desynchronized !== undefined) {
+            assert_equals(contextAttributes.desynchronized,
+                testScenario.expectedContextAttributes.desynchronized);
+        }
     }, testScenario.testDescription);
 }