Implement client side PBOs for glReadPixel

gpu/gpu.gyp

Index: gpu/gpu.gyp
diff --git a/gpu/gpu.gyp b/gpu/gpu.gyp
index 1bb1105c0f8b845bda577d4bcb7664f669bb13cc..d0611f5ba311e849dd930595705707b0b97fc3dc 100644
--- a/gpu/gpu.gyp
+++ b/gpu/gpu.gyp
@@ -263,6 +263,7 @@
         'command_buffer/tests/gl_virtual_contexts_unittests.cc',
         'command_buffer/tests/gl_stream_draw_unittests.cc',
         'command_buffer/tests/occlusion_query_unittests.cc',
+        'command_buffer/tests/gl_readback_unittests.cc',
       ],
       'conditions': [
         ['OS == "android" and gtest_target_type == "shared_library"', {