document chromium_src directory

docs/development/source-code-directory-structure.md

@@ -30,7 +30,7 @@ Electron
 |       loop into Chromium's message loop.
 |       └── api/ - The implementation of common APIs, and foundations of
 |           Electron's built-in modules.
-├── chromium_src/ - Source code that copied from Chromium.
+├── chromium_src/ - Source code copied from Chromium. See below.
 ├── default_app/ - The default page to show when Electron is started without
 |   providing an app.
 ├── docs/ - Documentations.
@@ -47,6 +47,19 @@ Electron
     components like `node` and `breakpad`.
 ```
 
+## `/chromium_src`
+
+The files in `/chromium_src` tend to be pieces of Chromium that aren't part of 
+the content layer. For example to implement Pepper API, we need some wiring 
+similar to what official Chrome does. We could have built the relevant 
+sources as a part of [libcc](../glossary.md#libchromiumcontent) but most 
+often we don't require all the features (some tend to be proprietary, 
+analytics stuff) so we just took parts of the code. These could have easily 
+been patches in libcc, but at the time when these were written the goal of 
+libcc was to maintain very minimal patches and chromium_src changes tend to be 
+big ones. Also, note that these patches can never be upstreamed unlike other 
+libcc patches we maintain now.
+
 ## Structure of Other Directories
 
 * **script** - Scripts used for development purpose like building, packaging,