UVC Compliant H264 Encoder for PAL/NTSC Analog Camera Modules
I'm not aware of any single-chip implementations of what you're looking for, but a three-chip solution can be put together quite easily.

The first step is to convert the analog video to a digital representation. BT.656 is the standard for digital representation of NTSC and PAL signals. There are many chips that can do this conversion; I'm most familiar with Analog Devices products in this area.

The second step is to encode the BT.656 signal with H.264. I'm not as familiar with products in this area, but I know for sure that Texas Instruments has chips that implement this.

The final step is to provide a high-speed USB device interface for the H.264-encoded data. There are many ways to accomplish this. I would start by looking at products from FTDI, but if their chips can't easily be interfaced to the encoder, then there are many microcontrollers with built-in USB hardware that could be programmed to do the job.