Fix a race condition in the unpacking of the extension system.
Fix a race condition in the unpacking of the extension system.

After asking for an extension to be unpacked, sometimes the
temp directory gets cleaned up before the unpacker can do its
work (in LoadExtensionsFromInstallDirectory, because the dir
contains no current version).

BUG=None
TEST=Add a reg key to HKLM to install an external extension
and make sure the extension gets installed.

Review URL: http://codereview.chromium.org/113795

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@16884 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,adobe/chromium,adobe/chromium,ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium