Postprocessing Volume properties change on changing the properties in its preset asset and vice versa.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Post Processing > Install.
3. Create a cube > Add Component > Post-process Volume.
4. Create a new Volume Profile > Add effects > Unity > Bloom
5. Create a preset for Post-process Volume.
6. Check all the properties checkbox and edit in Volume preset asset.

Actual Result:
Postprocessing Volume properties changes on changing the properties in Volume preset asset

Expected Result:
Postprocessing Volume properties should not be changed.

Tested in Package Version: 2.1.7, 2.1.6, 2.1.5, 2.1.4, 2.1.3, 2.1.2, 2.1.1, 2.1.0.

Reproducible on:
2020.1.0a12, 2019.3.0b10, 2018.2.0f2, 2018.1.0b5.

Environment:
Windows 10 & Mac 10.14.