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
  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)

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".

By default, components can depend only on the lower layers of the Chromium
codebase (e.g. base/, net/, etc.). Individual components may additionally allow
dependencies on the content API and IPC; however, if such a component is used
by Chrome for iOS (which does not use the content API or IPC), the component
will have to be in the form of a layered component
(http://www.chromium.org/developers/design-documents/layered-components-design).

Components that have bits of code that need to live in different
processes (e.g. some code in the browser process, some in the renderer
process, etc.) should separate the code into different subdirectories.
Hence for a component named 'foo' you might end up with a structure
like the following (assuming that foo is not used by iOS and thus does not
need to be a layered component):

components/foo          - DEPS, OWNERS, foo.gypi
components/foo/browser  - code that needs the browser process
components/foo/common   - for e.g. IPC constants and such
components/foo/renderer - code that needs renderer process

These subdirectories should have DEPS files with the relevant
restrictions in place, i.e. only components/*/browser should
be allowed to #include from content/public/browser.

Note that there may also be an 'android' subdir, with a Java source
code structure underneath it where the package name is
org.chromium.components.foo, and with subdirs after 'foo'
to illustrate process, e.g. 'browser' or 'renderer':

components/foo/android/OWNERS, DEPS
components/foo/android/java/src/org/chromium/components/foo/browser/
components/foo/android/javatests/src/org/chromium/components/foo/browser/

Code in a component should be placed in a namespace corresponding to
the name of the component; e.g. for a component living in
//components/foo, code in that component should be in the foo::
namespace.