Powering Arduino Mega 2560 from a Raspberry Pi Usb
If you are powering your RPi from power supply 5V/2A at least there should be no problem. USB hub on the RPi should not give more than 500mA to power Arduino. Also there is a fuse on Arduino's board to prevent any damage from USB power. 

Check both Arduino and RPi schematics for more details about modules used there:


Arduino
Raspberry