Fix crash with --load-extension for extensions with plugins

Ensure the extension install UI has a profile available to use for image loading, even if the prompt is opened before any browser window are available.

This bug is currently crashing Clusterfuzz ~100 times per hour and polluting the crash statistics.

BUG=344199
TEST=see bug
TBR=miket@chromium.org

Review URL: https://codereview.chromium.org/168823004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252077 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252077