20962 Split out EmbedderTest{Context,Compositor} to handle software and GL separately (cla: yes, waiting for tree to go green)

20972 Added keyEvent support for iOS 13.4+  (cla: yes, platform-ios, waiting for tree to go green)

21059 Add a new raster status kSkipAndRetry frame (cla: yes, platform-android, platform-ios, waiting for tree to go green)

21062 Make drain() consistently asynchronous. (cla: yes, waiting for tree to go green)

21074 Roll Skia from fb5e0ebef07c to 0b6bf1c9668e (2 revisions) (cla: yes, waiting for tree to go green)

21076 add a line to build host for scenario tests (cla: yes, waiting for tree to go green)