19848 Cherry pick 3d104f43649d333492dfa792968c8a09af93a84c (cla: yes, platform-android, platform-ios)

19859 Clipboard hasStrings method on iOS (cla: yes, platform-ios, waiting for tree to go green)

19919 Ensure threads are merged when tearing down the Rasterizer (cla: yes, platform-ios, waiting for tree to go green)

19962 Use the GrDirectContext factories instead of deprecated GrContext ones (cla: yes, platform-android, platform-ios)

20050 Use a single mask view to clip platform view (cla: yes, platform-ios)

20082 Ios nv12 external texture (cla: yes, platform-ios)