* Move task_observers.{cc,h} into fml.

This is a preparatory CL. The next CL will cause task_observers.h to
be included from both shell/platform/fuchsia _and_
fml/platform/fuchsia. I imagine it might be gauche for a single file
to be used from both those directories, but it seems way worse for fml
to depend on shell, rather than the other way around.

* Replace flutter_runner::Thread with fml::Thread.

This CL makes the treading logic in Fuchsia's flutter_runner more
consistent with fml expectations, but it still has quirks. Notably:

- Not all async work get posted to a fml::TaskRunner. Some work done
  by Fuchsia libraries gets posted directly to
  async_get_default_dispatcher(). This work doesn't trigger the
  fml::MessageLoop's task observers. As a result, we continue to have
  Fuchsia-specific task observers which fire for all async work,
  regardless of which way it was posted.

- There's awkwardness when trying to run Fuchsia code on a specific
  fml::TaskRunner if that fuchsia code accepts an
  async_dispatcher_t. Since you can no longer get an
  async_dispatcher_t for a given thread, you instead must post a
  closure to the fml::TaskRunner that calls
  async_get_default_dispatcher(), and then calls the fuchsia library
  with the default dispatcher.

- Some tests still use task_runner_adapter.h because async::Loop
  offers more control in unit tests than fml::MessageLoop does.

- If this successfully lands, there will be some cosmetic follow-up
  changes to make, like using ThreadHost instead of an array of
  fml::Threads.

* Increase stack size in flutter runner.

The previous change removes our custom 1MiB limit on the stack of
newly created threads, so these threads revert to the Fuchsia
default. google-internal tests were failing (without any particular
error in the logs) after this change, so here we just set the default
stack size to 1MiB.
Co-authored-by: NHunter Freyer <hjfreyer@google.com>

This reverts commit 05ce70eb.

* [web] replace browser-related conditional logic with BrowserEnvironment

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