Provide a way to signal that Perfetto will not be initialized

There is currently no way to signal that Perfetto will not be initialized in a given process.

This means that all calls to Perfetto must be guarded by `Tracing::IsInitialized()` or cause crashes if Perfetto is uninitialized.

In particular, any calls to [`Track::FromPointer`](https://github.com/google/perfetto/blob/main/include/perfetto/tracing/track.h#L120) must be guarded by this check.

I am currently working on an experiment to conditionally not initialize Perfetto in WebView, but this is blocked by the numerous call sites to `Track::FromPointer` (and other API methods) which are currently not guarded by `IsInitialized()`.

I propose to add a `Tracing::WillNotInitialize()` function to signal that Perfetto will not be initialized. With this, it would be safe to create otherwise invalid `Track` objects. Calls to `Tracing::Initialize()` should naturally be prohibited if `WillNotInitialize()` has previously been called.

Such a method will allow most call sites to Perfetto to avoid having to explicitly check for initialization.

Start with include/perfetto/tracing/track.h, especially Track::FromPointer, and trace the existing Tracing::IsInitialized() and Tracing::Initialize() APIs. Define how a new Tracing::WillNotInitialize() state affects invalid Track creation and later initialization, then verify the affected call sites and API behavior with the relevant tests.

Written by the indexing model from the issue text.