Merge public/common docs with the blink/public ones

Move //third_party/blink/public/common/README.md into
//third_party/blink/public/README.md, putting it together with the
public/web and public/platform documentation.

Bug: None
Change-Id: I560e1c419939ae026f1704d0b1fa0597069cfb95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007746Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#733325}

third_party/blink/public/README.md

@@ -23,9 +23,18 @@ Organization
 
 The API is organized into two parts:
 
+  - public/common
   - public/platform
   - public/web
 
+The public/common directory contains public headers for the Web Platform stuff
+that can be referenced both from renderer-side and browser-side code, also
+from outside the Blink directory (e.g. from `//content` and `//chrome`).
+Anything in this directory should **NOT** depend on other Blink headers.
+Corresponding .cc code normally lives in `blink/common`, and public `.mojom`
+files live in `blink/public/mojom`.  See `DEPS` and `blink/common/README.md`
+for more details.
+
 The public/platform directory defines an abstract platform upon which Blink
 runs. Rather than communicating directly with the underlying operating system,
 Blink is designed to run in a sandbox and interacts with the operating system

third_party/blink/public/common/README.md

-# Blink 'common' public directory
-
-This directory contains public headers for the Web Platform stuff that can
-be referenced both from renderer-side and browser-side code, also from
-outside the Blink directory (e.g. from `//content` and `//chrome`).
-
-Anything in this directory should **NOT** depend on other Blink headers.
-
-Corresponding .cc code normally lives in `blink/common`, and public
-`.mojom` files live in `blink/public/mojom`.
-
-See `DEPS` and `blink/common/README.md` for more details.