The A8-7600 relies on the DDR3 RAM installed on the motherboard for graphics tasks. Increasing dedicated memory on the an APU A8-7600 reduces the amount of RAM available to the rest of the system. For this reason, I recommend that you use at least 8GB of DDR3-2133MHz dual channel memory with the A8-7600.
The dedicated memory for the processor graphics can be increased in your BIOS on your motherboard. I would think this would make little, if any, noticeable difference in performance. The Radeon R7 A8-7600 graphics are typical weak performance compared to a discrete video card.
http://www.videocardbenchmark.net/gpu.php?gpu=Radeon+R7+A8-7600
As far as gaming, it looks as though the System Requirements for Watch Dogs are an actual discrete video card.
https://support.ubi.com/en-gb/Faqs/000019073/System-requirements-for-Watch-Dogs
If your question is about shared versus dedicated memory for video, I don't believe changing the amount of memory on weak processor graphics would make any noticeable difference. If you aren't using DDR3-2133MHz memory and don't set the memory parameters in the BIOS, (memory clock, DRAM Timings, DRAM voltage), then your performance may suffer.