WIP Canvas 2D/WebGL context: s/lowLatency/desynchronized/

This CL renames s/lowLatency/desynchronized/ on web-exposed parts
(s/low_latency/desynchronized/ internally) following [1,2] PRs
(issue [3]), and extends wpt getContextAttributes.html.

[1] whatwg/html#4360
[2] KhronosGroup/WebGL#2753
[3] whatwg/html#4087

Bug: 944199
Change-Id: Ic9d7bd6165f9dc9536613dc63e77b86b8ec42f7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532920
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643323}

2dcontext/context-attributes/getContextAttributes.html

@@ -6,22 +6,34 @@
 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: true",
+        canvasContextAttributes: {desynchronized: true},
+        expectedContextAttributes: {desynchronized : true}},
+    {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);
 }