Removing Page.WaitToLoad and update all pagesets to use explicit wait actions

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 7581ff800b6325282c269c2eb97e563b12d72fed..08586a3c8a217d40ad3f7cca277cbf1870fda5d0 100644
--- a/content/test/gpu/page_sets/pixel_tests.json
+++ b/content/test/gpu/page_sets/pixel_tests.json
@@ -4,7 +4,10 @@
   "pages": [
     {
       "url": "file:///../../data/gpu/pixel_webgl.html",
-      "wait_seconds": 4
+      "navigate_steps": [
+        { "action": "navigate"},
+        { "action": "wait", "condition": "duration", "seconds": 4 }
+      ]
     }
   ]
 }