Update README.md for Font Access API.

Change-Id: Ic1be80a3e7144327fe0ddc040fa2eb2bbe95d074
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3564616
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Daseul Lee <dslee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#988128}

content/browser/font_access/README.md

@@ -0,0 +1,38 @@
+# Font Access API
+
+This directory contains the browser-side implementation of
+[Font Access API](https://github.com/WICG/local-font-access/blob/main/README.md).
+
+## Related directories
+
+[`//third_party/blink/renderer/modules/font_access/`](../../../third_party/blink/renderer/modules/font_access/)
+contains the renderer-side implementation, and
+[`//third_party/blink/public/mojom/font_access`](../../../third_party/blink/public/mojom/font_access)
+contains the mojom interface for this API.
+
+## Code map
+
+It consists of the following parts:
+
+ * `FontAccessManager`: `content::FontAccessManager` implements
+   `blink::mojom::FontAccessManager`, providing a way to enumerate local fonts.
+   It checks for the requirements to access local fonts such as user permission,
+   page visibility and transient user activation. Once all the requirements are
+   met, it returns the fonts data from `FontEnumerationCache`. In terms of code
+   ownership, there is one `FontAccessManager` per `StoragePartitionImpl`;
+   `Frame`s are bound to `FontAccessManager` via a `BindingContext`.
+
+ * `FontEnumerationCache`: `content::FontEnumerationCache` is a cache-like
+   object that memoizes the data about locally installed fonts as well as the
+   operation status, given the user locale. The data is read via
+   `content::FontEnumerationDataSource.GetFonts()` and is stored and served in
+   `ReadOnlySharedMemoryRegion`. This class is not thread-safe. Each instance
+   must be accessed from a single sequence, which must allow blocking.
+
+ * `FontEnumerationDataSource`: `content::FontEnumerationDataSource`, inherited
+   by OS-specific subclasses, reads locally installed fonts from the underlying
+   OS. While the subclasses of this class contains the implementation for
+   reading font data, this class itself does not, as it is instantiated for
+   non-supported OS. It is not thread-safe; all methods except for the
+   constructor must be used on the same sequence, and the sequence must allow
+   blocking I/O operations.

third_party/blink/renderer/modules/font_access/README.md

@@ -1,28 +1,29 @@
 # Font Access API
 
-This directory contains the implementation of the Font Access API.
+This directory contains the renderer-side implementation of
+[Font Access API](https://github.com/WICG/local-font-access/blob/main/README.md).
 
 ## Related directories
 
-[`//third_party/blink/renderer/platform/fonts`](../../modules/font_access)
-contains the platform implementation.
+[`//content/browser/font_access/`](../../../../../content/browser/font_access/)
+contains the browser-side implementation, and 
+[`blink/public/mojom/font_access`](../../../public/mojom/font_access) contains the
+mojom interface for this API.
 
 ## APIs In this directory
 
-This directory contains the implementation of the new and still under
-development [Font Access API](https://github.com/inexorabletash/font-enumeration/blob/master/README.md).
-
 It consists of the following parts:
 
- * `FontMetadata`: Metadata about a font installed on the system. Provides
-   information that uniquely identifies a font, as well as text to present to
-   users in their system-configured locale.
-
- * `FontIterator`, `FontIteratorEntry`: Mechanism providing the ability to
-    enumerate system-installed fonts using an asynchronous iterator.
+ * `NavigatorFonts`: implements `navigator.fonts`.
 
- * `FontManager`: An object that exposes operations to access fonts. In the
-   future, this may be extended to add more capabilities.
+ * `FontManager`: An object that exposes an operation to access local fonts via
+   `query()` with an optional param `QueryOptions`. `FontManager` maintains a
+    mojo connection with browser-side, and is responsible for handling errors,
+    and optionally applying query filters provided by `QueryOptions`.
 
- * `NavigatorFonts`: An entry point, available in documents and workers on
-   `navigator`, that lets a website enumerate locally installed system fonts.
+ * `FontData`: Implemented as `FontMetadata.cc`, `FontData` represents data
+    about a single font installed on the system. Currently, it provides basic
+    information such as postscript name, full name, family and style;
+    addiitonally, it exposes a method `blob()` for accessing additional font
+    blob data via Blink's `FontCache`.
+    (See more details [here](../../../blink/renderer/platform/fonts))