All modern OSs will initialize the memory because there is a clear security issue with allowing one application access to the old contents of a random block of memory. It could contain passwords or who knows what else.
On the other hand, GPU memory is primarily used for rendering graphics. The security implications are less severe if information leaks. Has there ever been any guarantee information won't leak? So why do users assume that it won't? It is likely NOT cleared for speed reasons. Everyone wants a fast GPU cheap. Well, that's one way to get speed boost on allocation operations I'm sure.
It really should be Chrome's responsibility to zero out their GPU memory when they're done with it, to prevent information leakage. That's what incognito mode does with regards to history, cache, and cookies, so why not GPU memory?