[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

• gin/arguments.h[diff]
• gin/context_holder.h[diff]
• gin/dictionary.h[diff]
• gin/modules/console.h[diff]
• gin/modules/file_module_provider.h[diff]
• gin/modules/module_runner_delegate.h[diff]
• gin/per_context_data.h[diff]
• gin/per_isolate_data.h[diff]
• gin/public/isolate_holder.h[diff]
• gin/runner.h[diff]
• gin/test/file_runner.h[diff]
• gin/test/gtest.h[diff]
• gin/test/v8_test.h[diff]
• gin/try_catch.h[diff]
• gin/wrappable.h[diff]