Revert "Cache ShellImpl by resolved URL, not initial URL"

This reverts commit 8c71ea8f07989d102ea575f4d75947392900c414.
It broke tests: http://build.chromium.org/p/client.mojo/builders/Mojo%20Linux/builds/3

TBR=aa@chromium.org

Review URL: https://codereview.chromium.org/685013003