* Rename hittable to hitTestable

Apparently hittable is not politically correct.

* Enable tracing on Fuchsia

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.

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.