20468 Refactor initial route code take 2 (cla: yes, platform-ios)

20470 [web] update chrome version (cla: yes, waiting for tree to go green)

20479 Update to android 30r2 and roll buildroot (cla: yes)

20480 Flutter 1.21 candidate.10 (cla: yes)

20486 Remove the dummy rasterizer delegate now that flutter_runner is in tree, and cleanup ctor params (cla: yes, platform-android, platform-fuchsia, platform-ios, waiting for tree to go green)

20487 Only create raster_thread_merge when explicitly requested by the embedding platform (cla: yes, platform-ios, waiting for tree to go green)

20488 Fix unoptimized fuchsia builds (cla: yes)