9436 Add the functionality to merge and unmerge MessageLoopTaskQueues

9439 Eliminate unused import in FlutterView

9452 Convert RRect.scaleRadii to public method

9456 Made sure that the run_tests script returns the right error code.

9459 Remove unused/unimplemented shell constructor

9460 Fixed logLevel filter bug so that filter now works as expected.

9461 Adds API for retaining intermediate engine layers

9463 Removed unused imports in new embedding.

9466 Re-enable the Wuffs GIF decoder

9468 Manually draw remainder curve for wavy decorations

9485 Add –observatory-host switch

9486 Rework image & texture management to use concurrent message queues.