Charging an 8 V, 100 mA device via USB (5 V, 500 mA)
What will happen if you connect 5V directly to the shaver? Probably nothing bad. Most likely nothing at all: it won't charge. If I had to speculate (and I do) there's two 3.8V lithium cells in series in this shaver, so to charge these using a cheap linear regulator you need at least 7.6V, so 8V input can run that with a wee bit of margin.

It's possible to convert USB 5V to shaver 8V using a DC-DC step-up converter. 100mA is a modest current so it should be easy to build... or buy (see below.)

Here's one on eBay, but you may find others that are more suitable: https://www.ebay.com/itm/USB-DC-DC-5V-to-6V-7V-8V-9V-12V-15V-Adj-Step-up-Boost-Converter-UPS-Power-Supply-/172143724776

Here's another: https://www.diymore.cc/products/dc-dc-usb-5v-to-6-15v-step-up-boost-converter-voltage-inverters-module-adjustable-board-output-dc-6v-7v-8v-9v-12v

And here's a cord that claims to be the very thing you're seeking: https://www.amazon.com/Charger-Philips-Norelco-Replacement-Oneblade/dp/B081XX24M9

Happy shopping... or making.



Here is an example of an IC that would allow you to make your own boost converter with a very few additional components - TPS61046 datasheet here. The main disadvantage of this IC is the very small package it is supplied in.





If modest efficiency was acceptable then the ancient but very flexible and easy to use MC34063 - datasheet here $US0.59 in 1's from Digikey in 8 pin DIP package. Note the much larger inductor due to the much lower operating frequency and other example design factors. See datasheet for (easy) design guidelines. A sample PCB layout is provided. 

From fig 13 in the data sheet.
R1 R2 again set Vout.
C3 sets clock frequency. 
The 0.22 Ohm resistor is an optional current limit sensor.