Fuel efficiency is drive on the A350 XWB design, so aerodynamics and weight requirements are the first in the list.
Aerodynamics departments don't like flat surfaces in general. But there is also another key factor; visibility for pilots; they hate not being able to see.
Airbus made in 2008 a revision -that re-named the aircraft as XWB Xtra Wide Body- and the nose fuselage, that was similar to the A380, changed the shape of the upper radius and upper shell of the fuselage for aerodynamic reasons. Airbus refined the six-windscreen layout and worked to minimize the centre post to improve the pilots' visibility.
Glass makers –in this case Saint.Gobain- hate not-flat surfaces and the cost is higher because of complexity of the manufacturing process for a transparent laminate with rounded corners to prevent/mitgate against stress points in the cutout.
Structural engineers include an important and challenging requirement: to resist a birdstrike. The nose fuselage is one of the critical areas in the aircraft that must be certified against a bird impact test.
Manufacturability and birdstrike requirements defined the dimensions of the windows; smaller than a typical commercial jet and significantly smaller than windows on the 787, making them easier to certify for birdstrike and easier to manufacture as well.
In 2008 was also decided to remove the opening direct vision cockpit windows for flightcrew emergency evacuation and was included an escape hatch in the flightdeck roof instead; weight reduction and structural requirements optimization with low risk as it was the result of a trade off after benchmarking the configuration Boeing was using in the 787.
A big challenge in the 787 and A350 XWB going to curved windows is that curved windows are much harder than flat windows to optically match to the HUD (Head Up Display), which both these aircraft offer. The optical quality controls on the curved glass are extraordinarily tight, to be able to certify the HUD for takeoff and landing.
Finally, other tests like UV radiation, cold & hot temperature tests and ice balls firing tests are performed in the manufacturer facilities located at Sully sur Loire (France)