Made Pixel tests functional. Captures tab content and compares to ref image

content/test/gpu/page_sets/pixel_tests.json

Index: content/test/gpu/page_sets/pixel_tests.json
diff --git a/content/test/gpu/page_sets/pixel_tests.json b/content/test/gpu/page_sets/pixel_tests.json
index b96dd86d02fb465366a3d56101a2da82a9004bba..fead6853b246cc009c93b1288cf2d8468c72e6d4 100644
--- a/content/test/gpu/page_sets/pixel_tests.json
+++ b/content/test/gpu/page_sets/pixel_tests.json
@@ -3,11 +3,46 @@
   "user_agent_type": "desktop",
   "pages": [
     {
+      "name": "Pixel.Canvas2DRedBox",
+      "url": "file://../../data/gpu/pixel_canvas2d.html",
+      "navigate_steps": [
+        { "action": "navigate"},
+        {
+            "action": "wait",
+            "javascript": "domAutomationController._finished",
+            "timeout": 30
+        }
+      ],
+      "revision": 1,
+      "test_rect": [0, 0, 400, 300]
+    },
+    {
+      "name": "Pixel.CSS3DBlueBox",
+      "url": "file://../../data/gpu/pixel_css3d.html",
+      "navigate_steps": [
+        { "action": "navigate"},
+        {
+            "action": "wait",
+            "javascript": "domAutomationController._finished",
+            "timeout": 30
+        }
+      ],
+      "revision": 1,
+      "test_rect": [0, 0, 400, 300]
+    },
+    {
+      "name": "Pixel.WebGLGreenTriangle",
       "url": "file://../../data/gpu/pixel_webgl.html",
       "navigate_steps": [
         { "action": "navigate"},
-        { "action": "wait", "seconds": 4 }
-      ]
+        {
+            "action": "wait",
+            "javascript": "domAutomationController._finished",
+            "timeout": 30
+        }
+      ],
+      "revision": 1,
+      "test_rect": [0, 0, 400, 300]
     }
   ]
 }