You can join the GMail Search field trial to give this one a try. Google will now return content within your Google Mail and Google Drive accounts to the right of your Web search results. This is similar functionality to that of the cloud search tools like Cue and Cloudmagic.
If you have turned off search personalization, you need to turn it back on. You should also consider using an ad block extension, since the personalized search results will appear below the ads on the right side of the screen.
I am still playing around with this function. To be honest, I don’t have that much in my Mail or Drive accounts, since I tend to use other cloud storage services. I’m going to beef up my content in the Drive to see if I see any additional results. This will help me determine whether it’s problems with the personalized search or lack of content that is resulting in the very uneven appearance of Mail and Documents items.
On a somewhat unrelated note, I noticed when I was testing this feature out that Google is defaulting to returning Google Drive matches if you type a search that includes documents in it. This actually suits me better, since it means I can activate a Drive search just when I want it, rather than when Google thinks there’s something that matches.