Write down some design goals for WebLayer.

Change-Id: Ie7a38b77cfcc9a2c0ae6a7a5d5b64181e1614c9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919853
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718370}

weblayer/README.md

@@ -11,6 +11,14 @@ permissions, autofill, safe browsing etc...).
 While it's built on top of src/content, the expectation is that the API will
 hide the Content API.
 
+## Design Goals
+1. WebLayer should be powerful enough to build a modern browser. This means:
+    * as fast and secure as Chrome
+    * includes all the same web platform features as Chrome (including UIs and system integration code)
+2. WebLayer embedders should get new web platform features for free (e.g. don't need to keep updating their UIs or code)
+3. WebLayer hides subprocesses, so any code that needs to run in the renderer needs to be part of WebLayer. Script injection
+is generally discouraged for security and performance reasons.
+
 ## Resources and Documentation
 
 Mailing list: [weblayer-dev@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/weblayer-dev)