Command Buffer: read pixels into pixel pack buffer

gpu/command_buffer/client/gles2_implementation.cc

Index: gpu/command_buffer/client/gles2_implementation.cc
diff --git a/gpu/command_buffer/client/gles2_implementation.cc b/gpu/command_buffer/client/gles2_implementation.cc
index 587a5db07d5513758a194ac285f3bd7f0a1d1620..cebd672bef2b4beba173c48a44d9631844dd34da 100644
--- a/gpu/command_buffer/client/gles2_implementation.cc
+++ b/gpu/command_buffer/client/gles2_implementation.cc
@@ -3541,10 +3541,10 @@ void GLES2Implementation::ReadPixels(
   }
 
   if (bound_pixel_pack_buffer_) {
-    // TODO(zmo): Need to handle the case of reading into a PIXEL_PACK_BUFFER
-    // in ES3. For now, generate a GL error.
-    SetGLError(GL_INVALID_OPERATION, "glReadPixels",
-               "ReadPixels to a pixel pack buffer isn't implemented");
+    GLuint offset = ToGLuint(pixels);
+    helper_->ReadPixels(
+        xoffset, yoffset, width, height, format, type, 0, offset, 0, 0, false);
+    CheckGLError();
     return;
   }