18255 Restore integer snapping on OpacityLayer (cla: yes, perf: speed, severe: performance)

18439 Roll buildroot to flutter/buildroot@a4f3c4d5023e080ee50596e6623d179e9c5f839b (cla: yes, perf: app size, perf: speed, severe: performance, waiting for tree to go green)

18516 [profiling] Memory Profiling support for iOS (cla: yes, perf: memory, severe: performance)

18829 Correct BM_ShellInitializationAndShutdown (cla: yes, perf: speed, severe: performance, waiting for tree to go green)

18838 Avoid creating a vector when constructing Dart typed data objects for platform messages (cla: yes, perf: speed, severe: performance, waiting for tree to go green)