9797 Remove breaking asserts (cla: yes)

9799 Update buildroot to c4df4a7b to pull in MSVC 2017 Update 9 on Windows. (cla: yes)

9808 Document FontFeature class (cla: yes)

9809 Document //flutter/shell/common/rasterizer (cla: yes)

9813 Made Picture::toImage happen on the IO thread with no need for an onscreen surface. (cla: yes)

9816 Only release the image data in the unit-test once Skia has accepted ownership of it. (cla: yes)

9818 Convert run_tests to python, allow running on Mac/Windows and allow filters for tests. (cla: yes)

9823 Roll buildroot to support bitcode enabled builds for iOS (cla: yes)