* Make pipeline hold a deque so we can push_front

- Also assert that all the DoDraw tasks are executed
  on the GPU thread.

* Add tests for pipeline

* Add support for pipeline to push resources to the front

Added class docstrings for classes inside of shell/common.

* Revert "[fuchsia] Fix alignment of Fuchsia/non-Fuchsia tracing (#9289)"

This reverts commit f80ac5f5.

* Revert "Align fuchsia and non-fuchsia tracing (#9199)"

This reverts commit 78265484.

When flutter/synchronization was first authored, we did not own fml (it was called fxl then). Now we do, so use a single spot for such utilities. The pipeline was meant to be a general purpose utility that was only ever used by the animator (it even has animator specific tracing), so move that to shell instead (where the animator resides).

We're going to remove fxl::Mutex soon.

* Add trace event slices for the duration in which the engine produces a pipeline item.

* Trace the entire lifetime of an item in the pipeline.

Pipelines vend single use continuations that callers can complete at any time to signal readiness of a pipeline resource. (#2957)