Implement the ability to obliterate a storage partition from disk.

Implement the ability to obliterate a storage partition from disk.

On the uninstall of an extension with isolated storage, we want to delete
all the data for the extension from disk as soon as possible. Because we
cannot know when various objects with state on disk 
(eg., FileSystemContext) have all been deleted, we do a best-effort delete
for any directory that we know isn't being used.

The way this gets projected into the content modulue is that each extension
defines one partition_domain. If an extension has a <webview> tag, it will
also have multiple StoragePartitions, each with a different partition_name.
If it doesn't have a <webview> tag, the partition_name is considered empty
which yields the default partition. The default partition, and all webview
partitions are peers inside the partition_domain's root directory.

This CL introduces a function that allows us to delete partiton domain.
Special care is taken to not accidentally instantiate a StoragePartition for
the domain if none current exists. This is necessary to allow us to actually
delete the whole partition domain directory.

(Patch by ajwong@chromium.org)

BUG=85127
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=168405

[Charlie Reis, 2012-11-16 20:11:16]

Matt, can you repeat your review on this copy?  It's the same as
https://chromiumcodereview.appspot.com/11362268/ with the include order fixed in
storage_partition_impl.cc.  Thanks!

[Matt Perry, 2012-11-16 20:15:59]

lgtm

[commit-bot: I haz the power, 2012-11-16 20:18:08]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/creis@chromium.org/11280030/1

[commit-bot: I haz the power, 2012-11-17 12:20:12]

Change committed as 168405