Fix race condition in extension service that causes extensions installed concurently with the extension garbage collection to sometimes get marked for deletion.  The concurrency is between the extension installation in the browser process and the garbage collection in the import process.
Fix race condition in extension service that causes extensions installed
concurently with the extension garbage collection to sometimes get marked for
deletion.  The concurrency is between the extension installation in the browser
process and the garbage collection in the import process.

BUG=101935
TEST=Install google chrome and look at the default apps installed in the first
profile.  The icons for all the apps should be correct.  This does not work
with chromium builds since default apps are not installed there.

Review URL: http://codereview.chromium.org/8872031

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@114695 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium