Move ExtensionPrefs and friends to extensions/ directory.

Move ExtensionPrefs and related classes from
chrome/browser/extensions/ to the top-level extensions/
directory. The following classes are moved:

ExtensionPrefStore
ExtensionPrefValueMap
ExtensionPrefValueMapFactory
ExtensionPrefs
ExtensionPrefsFactory
ExtensionScopedPref

The TestExtensionPrefs helper class and the
ExtensionPrefsTest unit tests remain in
chrome/browser/extensions, since TestExtensionPrefs has a
dependency on Chrome's PrefServiceSyncable that seems like it
will be difficult to break (more explanation at
https://codereview.chromium.org/108643002/).

BUG=313284
TBR=sky@chromium.org,pam@chromium.org

Review URL: https://codereview.chromium.org/46853004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246373 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246373
Disable HistoryWebUITest.basicTest, it times out.

BUG=336845
NOTREECHECKS=true
NOTRY=true
BUG=None
TBR=dubroy@chromium.org

Review URL: https://codereview.chromium.org/136703006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246374 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246374