Add documentation of flow of information for Text-to-Speech on Chrome/CROS.

NOTRY=true
TBR=dmazzoni@chromium.org

Change-Id: I52c8cb2287f31554f5c6faab740ebd701b135eed
Reviewed-on: https://chromium-review.googlesource.com/1013693
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551355}

docs/README.md

@@ -280,6 +280,10 @@ used when committed.
     feedback (ChromeVox) on Chrome OS.
 *   [ChromeVox on Desktop Linux](accessibility/chromevox_on_desktop_linux.md) -
     Enabling spoken feedback (ChromeVox) on desktop Linux.
+*   [Offscreen, Invisible and Size](accessibility/offscreen.md) - How Chrome
+    defines offscreen, invisible and size in the accessibility tree.
+*   [Text to Speech](accessibility/tts.md) - Overview of text to speech in 
+    Chrome and Chrome OS.
 *   [BRLTTY in Chrome OS](accessibility/brltty.md) - Chrome OS integration with
     BRLTTY to support refreshable braille displays
 *   [PATTS on Chrome OS](accessibility/patts.md) - Notes on the PATTS speech

docs/accessibility.md

@@ -6,6 +6,7 @@
 ## Cross-Platform
 
 * [Offscreen, Invisible and Size](accessibility/offscreen.md)
+* [Text to Speech in Chrome and Chrome OS](accessibility/tts.md)
 
 ## Chrome OS
 

docs/accessibility/patts.md

@@ -3,6 +3,8 @@
 Chrome OS comes with a speech synthesis engine developed internally at Google
 called PATTS. It's based on the same engine that ships with all Android devices.
 
+[Read more about Text-to-Speech in Chrome](tts.md).
+
 ## Building from source
 
 This is for Googlers only.

docs/accessibility/tts.md

+# Text to Speech in Chrome and Chrome OS
+
+Chrome and Chrome OS allow developers to produce synthesized speech. This
+document is an overview of the relevant code and code structure around
+synthesized speech.
+
+## Code structure
+
+A brief outline of the flow from speech request to the resulting speech on any
+platform.
+
+### Input
+
+- chrome.tts extension API
+
+    - The [chrome.tts extension API](https://developer.chrome.com/apps/tts)
+    allows extensions to request speech across Windows, Mac or Chrome OS, using
+    native speech synthesis.
+
+    - Input to the extension is first processed in the
+    [TtsExtensionApi](https://cs.chromium.org/chromium/src/chrome/browser/speech/extension_api/tts_extension_api.h).
+
+    - The extension is passed an [Options object](https://developer.chrome.com/apps/tts#method-speak)
+    in chrome.tts.speak, which is translated into a
+    [tts_controller Utterance](https://cs.chromium.org/chromium/src/chrome/browser/speech/tts_controller.h?dr=CSs&l=130).
+
+- Web Speech API
+
+    - Chrome implements
+    [Window.SpeechSynthesis](https://developer.mozilla.org/en-US/docs/Web/API/Window/speechSynthesis)
+    from the [Web Speech API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Speech_API).
+    This allows web apps to do text-to-speech via the device's speech
+    synthesizer.
+
+    - A [WebSpeechSynthesisUtterance](https://cs.chromium.org/chromium/src/third_party/blink/public/platform/web_speech_synthesis_utterance.h)
+    is created by window.SpeechSynthesis
+
+### Processing
+
+- The [TtsController](https://cs.chromium.org/chromium/src/chrome/browser/speech/tts_controller.h) processes utterances and sends them to the correct output engine
+
+### Output
+
+- May differ by system, including Mac, Wind, Android, Arc++, and Chrome OS
+
+- In Chrome OS:
+
+    - [TtsEngineExtensionAPI](https://cs.chromium.org/chromium/src/chrome/browser/speech/extension_api/tts_engine_extension_api.h)
+    forwards speech events to PATTS, or the network speech engine, or,
+    coming soon, third-party speech engines.
+
+    - [PATTS](patts.md) is the built-in Chrome OS text-to-speech engine.