This patch adds a default shader warm up process which moves shader compilation from the animation time to the startup time. This also provides an extension for `runApp` so developers can customize the warm up process.

This should reduce our worst_frame_rasterizer_time_millis from ~100ms to ~20-30ms for both flutter_gallery and complex_layout benchmarks. Besides, this should also have a significant improvement on 90th and 99th percentile time (50%-100% speedup in some cases, but I haven't tested them thoroughly; I'll let our device lab collect the data afterwards).

The tradeoff the is the startup time (time to first frame). Our `flutter run --profile --trace-startup` seems to be a little noisy and I see about 100ms-200ms increase in that measurement for complex_layout and flutter_gallery. Note that this only happens on the first run after install or data wipe. Later the Skia persistent cache will remove the overhead.

This also adds a cubic_bezier benchmark to test the custom shader warm up process.

This should fix https://github.com/flutter/flutter/issues/813 (either by `defaultShaderWarmUp`, or a `customShaderWarmUp`).