extensions: add overview doc

Bug: None
Change-Id: I22230d6e321f38c5870ef0a02770d4b74ec36472
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031743Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738645}

extensions/docs/overview.md

+# Extensions Overview
+
+This document is an overview of extensions concepts, terms, and architecture.
+The target audience is engineers working on extension support code in the
+browser, *not* extension authors. See also the [public extensions
+documentation].
+
+## Glossary
+
+* **Extension**: a bundle of HTML, CSS, and Javascript that can interact with
+  web contents and some parts of the browser UI. There are many types of
+  extensions; see [extension types] for more details. Each extension has an
+  **id**, which is a unique string identifying that extension.
+* **Manifest**: a JSON file stored inside the extension bundle which describes
+  the extension, the capabilities it needs, and many other things about it. The
+  [manifest file format] doc gives a user-facing overview.
+* **Action**: an action is one of the ways an extension can expose
+  functionality to a user. There are three types: [page actions],
+  [browser actions], and regular actions. Actions add buttons to the toolbar or
+  to the extension menu.
+* **Permission**: the ability of an extension to access or not access a specific
+  API. Most things extensions can do are controlled by permissions. See
+  [permissions] for more details.
+* **Extension renderer**: because extensions are logically their own web
+  environments, each extension may have a renderer process that hosts its
+  content. These renderers are annotated in the task manager as
+  "Extension: Name".
+* **Component extension**: an extension that is part of the browser. Component
+  extensions are not user-visible, generally can't be configured or disabled,
+  and may have special permissions. A component extension is so named because it
+  is logically a *component of the browser* that happens to be an extension.
+
+## Key Classes
+
+### [extensions::Extension]
+
+An instance of class Extension represents a single installed extension.
+Instances of this class are mostly immutable, and are often passed around as
+`scoped_refptr<const Extension>`.
+
+### [extensions::Manifest]
+
+An instance of this class represents the manifest of an Extension. An Extension
+has (and owns) exactly one Manifest. The Manifest contains the parsed JSON data
+from an extension's manifest.json.
+
+### [extensions::ExtensionSystem]
+
+An instance of this class manages much of the state needed to manipulate and use
+extensions. It controls many subsidiary services and controllers. It also
+contains an [extensions::ExtensionService], which is a mostly-historical grab
+bag that is still used for many operations.
+
+### [extensions::Extension::ManifestData] and [extensions::ManifestHandler]
+
+TODO(ellyjones): Move this to a separate manifest.md doc?
+
+These two cooperating classes allow for manifest parsing to be modular. An
+instance of ManifestHandler receives the manifest and parses data out of it as
+desired, attaching data to the Extension in question via
+`extensions::Extension::SetManifestData`. If you were adding a new field to the
+manifest, you would:
+
+1. Create a new ManifestData subclass to store whatever you need to store
+   per-Extension
+2. Create a new ManifestHandler subclass to parse an Extension's manifest
+3. Register the class from (2) in `extensions::RegisterChromeManifestHandlers`
+4. Grab the ManifestData out of the Extension via
+   `extensions::Extension::GetManifestData` as needed. Conventionally, one adds
+   a static method to the ManifestData subclass from (1) to retrieve it from an
+   Extension.
+
+## Sync
+
+TODO(ellyjones): How does extension sync work?
+
+## Extension Process Model
+
+TODO(ellyjones): Write some words!
+
+[background page]: https://developer.chrome.com/extensions/background_pages
+[browser actions]: https://developer.chrome.com/extensions/browserAction
+[builder pattern]: https://en.wikipedia.org/wiki/Builder_pattern
+[chrome.commands]: https://developer.chrome.com/extensions/commands
+[chrome.omnibox]: https://developer.chrome.com/extensions/omnibox
+[extension types]: extension_and_app_types.md
+[manifest file format]: https://developer.chrome.com/extensions/manifest
+[page actions]: https://developer.chrome.com/extensions/pageAction
+[permissions]: permissions.md
+[public extensions documentation]: https://developer.chrome.com/extensions
+
+[extensions::Extension::ManifestData]: https://cs.chromium.org/chromium/src/extensions/common/extension.h
+[extensions::ExtensionService]: https://cs.chromium.org/chromium/src/chrome/browser/extensions/extension_service.h
+[extensions::ExtensionSystem]: https://cs.chromium.org/chromium/src/extensions/browser/extension_system.h
+[extensions::Extension]: https://cs.chromium.org/chromium/src/extensions/common/extension.h
+[extensions::ManifestHandler]: https://cs.chromium.org/chromium/src/extensions/common/manifest_handler.h
+[extensions::Manifest]: https://cs.chromium.org/chromium/src/extensions/common/manifest.h