* Rename hittable to hitTestable

Apparently hittable is not politically correct.

* Enable tracing on Fuchsia

This requires an updated buildroot.

After this patch, we no longer link with Mojo. We still use some gn definitions
from //mojo to create Dart packages.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

Previously we had a mix of the two.

This updates use of mtl::DataPipeDrainer to start reading in a separate
method.

This fixes this Fuchsia build, which uses the latest //lib/ftl.

This patch depends on another patch that hasn't landed yet.

This reverts commit 960da66f.

* Wire up to mojo::tracing::* infrastructure.

* Initialize tracing as early as possible.

* Make dependency specific to fuchsia.

We were creating the task runner adaptor before the underlying base::Thread had
a TaskRunner to adapt. Now we wait until after we start the thread.

This patch centralizes the asset bundle processing in //flutter/assets and
removes the //base dependency from the code.

We don't have a way to print stack traces on Fuchsia yet. This patch
isolates this base dependency in //glue.

The ftl::TaskRunner is still based on a base::MessageLoop, but this patch paves
the way to replace the message loop on Fuchsia.

This patch introduces a //glue library that isolates our dependency on //base.
This will let us clean up our //base dependencies ahead of actually being able
to fully remove them.