[DRAFT] Add new default stylesheet for platform apps.

[DRAFT] not suitable for review/commit.

This CL is dependent on http://codereview.chromium.org/8429023/, which
at the time of this writing isn't yet approved.

BUG=none
TEST=none

[Aaron Boodman, 2011-11-06 23:28:41]

I don't think you want to add any code to the browser process.

The list of extensions is available in the renderer, too [1], so you can just
iterate the set of extensions, find the ones that are platform apps, and add
this new stylesheet using their url() as the basis for the pattern.

The list of extensions can be changed at runtime, so you will need to listen for
ExtensionMsg_Loaded and ExtensionMsg_Unloaded in order to call
WebView::addUserStyleSheet with the right values.

Note that I think there can be only one handler for each IPC message and we
already handle these in ExtensionDispatcher, so maybe my advice to create an
app-specific dispatcher was off, at least for now. Maybe what you really want is
a little PlatformAppStylesheetController object or something which
ExtensionDispatcher delegates to. 

[1] You will need to add an entry here:
http://git.chromium.org/gitweb/?p=chromium/chromium.git;a=blob;f=chrome/commo....