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

content/test/gpu/page_sets/pixel_tests.json

 {
   "description": "Some basic test cases for GPU.",
   "user_agent_type": "desktop",
   "pages": [
     {
+      "url": "file:///../../data/gpu/pixel_browser_plugin.html",
+      "navigate_steps": [
+        { "action": "navigate"},
+        { "action": "wait", "seconds": 2 }

[piman, 2013/09/04 19:31:24]

Rather than wait an arbitrary number of seconds (and risk flakiness), could we
wait for a particular event coming from the page instead?

[Ken Russell (switch to Gerrit), 2013/09/05 00:27:04]

Agree 100%. While frame-synced snapshots are in development (Issue 256848),
these tests ought to be able to issue something like three no-op
requestAnimationFrame callbacks, after which the results should be guaranteed to
be on screen. Each test should import some JavaScript helper file to implement
this, and then that one file can be changed to change the tests' exit
conditions.

+      ],
+      "revision": 1
+    },
+    {
+      "url": "file:///../../data/gpu/pixel_canvas2d.html",
+      "navigate_steps": [
+        { "action": "navigate"},
+        { "action": "wait", "seconds": 2 }
+      ],
+      "revision": 1
+    },
+    {
+      "url": "file:///../../data/gpu/pixel_css3d.html",
+      "navigate_steps": [
+        { "action": "navigate"},
+        { "action": "wait", "seconds": 2 }
+      ],
+      "revision": 1
+    },
+    {
       "url": "file:///../../data/gpu/pixel_webgl.html",
       "navigate_steps": [
         { "action": "navigate"},
-        { "action": "wait", "seconds": 4 }
-      ]
+        { "action": "wait", "seconds": 2 }
+      ],
+      "revision": 1
     }
   ]
 }