Use new showSnackbar() method everywhere.

This updates all the clients of showSnackbar() to use the new variant of
the method that takes a single Snackbar object. The old overloads of
showSnackbar() have been removed.

This also renames removeSnackbarEntry() to removeMatchingSnackbars(),
which seems like a clearer name.

Review URL: https://codereview.chromium.org/1235453002

Cr-Commit-Position: refs/heads/master@{#338390}
5 files changed