Add some missing README files

Change-Id: I307a15e13b577db6ab3b728f09fbce65e3e528d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1793408
Commit-Queue: Darin Fisher <darin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695202}

android_webview/README.md

+# Android WebView
+
+This directory contains the Android WebView implementation.
+
+See https://developer.android.com/reference/android/webkit/WebView for
+developer documentation.

chrome/README.md

+# Chrome
+
+This directory contains the open source, application layer of Google Chrome.
+
+Unlike other parts of Chromium like //content, which provide framework intended
+to support multiple products, this directory contains code that is focused on
+building specific products with opinionated UX.
+
+Specific products include:
+- Chrome desktop browser for Chrome OS, Windows, Mac and Linux
+- Chrome mobile browser for Android
+- Chrome OS system UI
+
+See //ios for the Chrome mobile browser for iOS, and note that code that is
+shared between //chrome and //ios is typically factored out into //components.

chromeos/README.md

+# Chrome OS
+
+This directory contains low-level support for Chrome running on Chrome OS. Many
+subdirectories contain Chrome-style C++ wrappers around operating system
+components.
+
+For example, //chromeos/dbus contains wrappers around the D-Bus interfaces to
+system daemons like the network configuration manager (shill). Most other
+directories contain low-level utility code. For example, //chromeos/disks has
+utilities for mounting and unmounting disk volumes.
+
+There are two exceptions:
+
+- //chromeos/services contains mojo services that were not considered
+  sufficiently general to live in top-level //services. For example
+  //chromeos/services/secure_channel bootstraps a secure communications channel
+  to an Android phone over Bluetooth, enabling multi-device features like
+  instant tethering.
+
+- //chromeos/components contains C++ components that were not considered
+  sufficiently general to live in top-level //components. For example,
+  //chromeos/components/account_manager manages the user's GAIA accounts, but
+  is used as the backend for UI that only exists on Chrome OS devices.
+
+Note, //chromeos does not contain any user-facing UI code, and hence it has
+"-ui" in its DEPS. The contents of //chromeos should also not depend on
+//chrome or //content.