25898 Revert “Fix a11y placeholder on desktop; auto-enable engine semantics… (cla: yes, platform-web)

25899 Ensure that AutoIsolateShutdown drops its reference to the DartIsolate on the intended task runner (cla: yes, waiting for tree to go green)

25900 Fix composition when multiple platform views and layers are combined  (platform-android, cla: yes, waiting for tree to go green)

25901 Don’t call release_proc when attempting to wrap an SkSurface which fails (cla: yes, embedder)

25904 Roll Skia from bf688645acf9 to 537293bf155f (1 revision) (cla: yes, waiting for tree to go green)