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
https://github.com/whatwg/html/pull/4360 (which addressed
https://github.com/whatwg/html/issues/4087).

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

third_party/blink/renderer/modules/canvas/htmlcanvas/canvas_context_creation_attributes_helpers.cc

@@ -19,7 +19,7 @@ CanvasContextCreationAttributesCore ToCanvasContextCreationAttributes(
   result.fail_if_major_performance_caveat =
       attrs->failIfMajorPerformanceCaveat();
 #if defined(OS_MACOSX)
-  // TODO(crbug.com/922218): enable desynchronized on Mac.
+  // TODO(crbug.com/945835): enable desynchronized on Mac.
   result.desynchronized = false;
 #else
   result.desynchronized = attrs->desynchronized();

third_party/blink/web_tests/external/wpt/2dcontext/context-attributes/getContextAttributes.html

@@ -6,13 +6,16 @@
 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) {
@@ -20,8 +23,14 @@ function runTestScenario(testScenario) {
         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);
 }