Implement special origin database handling for Isolated Apps

Implement special origin database handling for Isolated Apps

- Factor out SandboxOriginDatabase interface to SandboxOriginDatabaseInterface
- Implement SandboxIsolatedOriginDatabase which implements the same interface
  and only handles a single origin without database

BUG=242272
TEST=SandboxIsolatedOriginDatabaseTest.*
TEST=manually tested (launch Chrome w/o change, launch an app which uses HTML5 FS API, write some data -> launch Chrome with change, make sure the same contents appear & origin database's gone)
TBR=tzik@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=203139

[kinuko, 2013-05-29 05:31:32]

PTL

[tzik, 2013-05-29 08:38:24]

lgtm

[kinuko, 2013-05-30 02:44:19]

Changed the default origin directory name from "000" to "iso" (of 'isolated'),
as I wanted to make it more distinguishable from regular origin directories.
https://codereview.chromium.org/16150002/diff2/31001:44001/webkit/browser/fil...

TBR-ing jam@ for adding one test in content/content_tests.gypi

Also CC-ing ajwong@ as it's remotely related to the storage partition.  (We're
dropping origin database for origins that have isolated storage, i.e. when
extensions::AppIsolationInfo::HasIsolatedStorage(extension) is true)

[kinuko, 2013-05-30 13:04:24]

Committed patchset #2 manually as r203139 (presubmit successful).