Lifetime annotations

Overview

As part of the WebView Multi-profile project, classes and state in `//android_webview/`` will be annotated to make their lifetimes explicit. There are broadly 5 different lifetimes that a class can have:

Singleton - there is one instance of this class per WebView browser process. As WebView is run in the embedding app's process, this means there is one instance per embedding app.
Scoped to Profile - this class is specific to a Profile. It is likely to do with the user's browsing state, such as cookies or permissions.
Scoped to WebView - this class is specific to a WebView (the Android View), for example the AwZoomControls.
Temporary - this class has a shorter lifetime than a WebView. Examples include objects that live within a single call stack, objects associated with a single HTTP request or page navigation, etc.
Renderer - this class has a lifetime tied to specific to a Renderer.

There is a many to one relationship between WebViews and Profiles. A single Profile can support multiple WebViews, each WebView will only have a single (constant) Profile.

These annotations are purely for documentation, there is no static analysis to check them.

Annotations

In Java, use the following annotations:

@Lifetime.Singleton
@Lifetime.Profile
@Lifetime.WebView
@Lifetime.Temporary
@Lifetime.Renderer

In C++, use the following comment format as the last line of the class documentation:

// Lifetime: Singleton
// Lifetime: Profile
// Lifetime: WebView
// Lifetime: Temporary
// Lifetime: Renderer