//components: Sharpen message about code shared by Blink

Came out of an offline discussion about the ambiguity between placing
such code in //components or //third_party/blink/common.

Change-Id: Icc4f4d6edc2af78dfc145d2c5cb54caef7d5f04e
Reviewed-on: https://chromium-review.googlesource.com/1138155
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575627}

components/README

 This directory is for features that are intended for reuse. Example use cases:
--code that is shared by Chrome on iOS and other Chrome platforms (since the iOS
-  port doesn't use src/chrome)
--code that is shared between multiple embedders of content (e.g., Android
+- features that are shared by Chrome on iOS and other Chrome platforms (since
+  the iOS port doesn't use src/chrome)
+- features that are shared between multiple embedders of content (e.g., Android
   WebView and Chrome)
--code that is shared between Blink and the browser process (since code in the
-  browser doesn't use Blink, while Blink doesn't include content or chrome to
-  avoid circular dependencies)
+- features that are shared between Blink and the browser process
+  * Note: It is also possible to place code shared between Blink and the
+    browser process into //third_party/blink/common. The distinction comes
+    down to (a) whether Blink is the owner of the code in question or a consumer
+    of it and (b) whether the code in question is shared by Chrome on iOS as
+    well. If the code is conceptually its own cross-process feature with Blink
+    as a consumer, then //components can make sense. If it's conceptually
+    Blink code, then third_party/blink/common likely makes more sense. (In the
+    so-far hypothetical case where it's conceptually Blink code that is shared
+    by iOS, raise the question on chromium-dev@, where the right folks will see
+    it).
 
 In general, if some code is used by a directory "foo" and things above "foo" in
 the dependency tree, the code should probably live in "foo".