Correctly set is-form-webstore flag for ExternalCache downloads.

ExternalCache didn't actually check the update URL for cache entries
added via the download code path, resulting in is-from-webstore checks
yielding the wrong result in extension installation code. This
resulted in one of the demo extensions failing installation, which
subsequently caused the extension package to be purged from the cache.

BUG=chromium:274608
TEST=See bug.
R=dpolukhin@chromium.org

Review URL: https://codereview.chromium.org/25035003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225715