[Gin] Add documentation to explain how Gin works

This code is likely to evolve over time, but we've reached a stage where the
basic structure is mostly in place. This CL documents this structure so that
future developers will understand what we have in mind now.

R=aa@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/88913006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237485 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237485
Notify WebSocket handshake information to the renderer, chromium side

Notify WebSocket opening handshake request / response to the renderer process
to show them in the inspector.
Moved WebSocketHandshake{Request, Response} from modules/websockets
to platform/network to use them in platform/exported .

Blink side CL is: https://codereview.chromium.org/77273002/

BUG=310405
R=ricea, tsepez, jam

Review URL: https://codereview.chromium.org/77513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237486 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237486