WORK IN PROGRESS.

WORK IN PROGRESS.

Enables sync for extension settings.

BUG=To be filed
TEST=Unit tests

[not at google - send to devlin, 2011-08-26 09:58:46]

As I said, this is a work in progress, though quite close to being finished. 
There a few TODOs/XXXs I didn't get the chance to finish before the end of the
week, plus a puzzling test failure.

So this is an FYI.  Using the test server with 2 browsers (my linux and osx
workstations), sync seems pretty robust.

I can probably split this down a bit, e.g. submit the sync infrastructure
changes first (protobuf, new datatype, etc) and the meat of it later.

http://codereview.chromium.org/7747043/diff/2001/chrome/browser/ui/webui/ntp/...
File chrome/browser/ui/webui/ntp/ntp_resource_cache.cc (right):

http://codereview.chromium.org/7747043/diff/2001/chrome/browser/ui/webui/ntp/...
chrome/browser/ui/webui/ntp/ntp_resource_cache.cc:486: } else if
(unacknowledged.test(syncable::EXTENSION_SETTINGS)) {
I'm not sure what this is about but the CHECK on the line below gets triggered
without it, or at least, it was last week.  I'll look into it again when I get
the chance.

[Matt Perry, 2011-08-26 18:36:03]

I'm going to be out next week, and I think Ben is working on extension sync, so
he might be a better reviewer. Handing it off to him.

[not at google - send to devlin, 2011-08-29 09:06:18]

Don't look at this any more; look at http://codereview.chromium.org/7775008/
instead, the non-WIP (and synced to tip) version.