Add option to use integrated GPU for hardware acceleration

When hardware acceleration is enabled on machines with multiple GPUs like the higher-end MacBooks, Brave engages the discrete GPU, even when running on battery power.

This is not a major issue, however an option to always run on the integrated GPU can be useful to extend battery life and improve thermal performance.

This is implemented as a preference in Opera (their Chromium fork), independent of their other “battery saver” feature. From my experience, running on the iGPU is a net positive, without noticeable performance impact on any site but with significantly longer and cooler browsing sessions.

Edit: Upon further inspection, Brave steps up to the discrete GPU only when a WebGL context is created (e.g. Google Maps), but the problem is that it never switches back, even after all browser windows are closed. The “needs high perf GPU” flag stays on until I quit the browser.

Hardware: MacBook Pro (Retina, 15-inch, Mid 2015)
GPU: AMD Radeon R9 M370X + Intel Iris Pro Graphics 5200
Operating System: macOS Mojave (10.14.6)
Brave Version: 0.69.135 Chromium: 77.0.3865.120