Add command line flag to override gallery update urls

This will be helpful for testing installs via the web store pointing at a test
server, and potentially for testing autoupdate as well.

Also remove the kGalleryBrowsePrefix constant while I was in the neighborhood, 
since it turns out it's not used anywhere in our code anymore.

BUG=63511
TEST=Run chrome with --app-gallery-update-url=<SOMEURL>. Go to the web store
and try installing an app - the request to download the .crx file should go
<SOMEURL> with some extra parameters tacked on including the extension id,
etc. instead of http://clients2.google.com/service/update2/crx.



Review URL: http://codereview.chromium.org/5119003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67625 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed